radiocarbon chronology of the early human occupation of argentina this paper reports the compilation and initial analysis of a database of 14c dates from archaeological sites in argentina for the period 13 000 and 7000 14c bp the spatial and temporal distribution of human occupation evidence during this period is surveyed with discussion of potential sampling biases affecting the recognition of such sites in each region of argentina the archaeological signal of human occupation at the beginning of this period 13 000 11 000 14c bp is suggestive but sparse and weak however from about 11 000 14c bp there is a consistent archaeological signal most of the earliest occupation evidence comes from the eastern pampas and from southern patagonia in the chaco and northeast regions similarly early archaeological sites have not yet been detected this contrast may reflect the actual distribution of early human occupation but it may also reflect biased research effort and or systemic factors affecting the visibility and level of preservation of the early archaeological record in other regions such as the eastern slope of the andes in patagonia the peri andean central west and the northwest where there has been intensive archaeological research for an extended period and there are fewer systematic factors which are likely to bias archaeological visibility the absence of archaeological evidence of early occupation may indicate delayed colonization of these more marginal environments the conclusion discusses the implications of 14c dates associated with fishtail projectile points for the western hemisphere pre clovis debate and the implications of terminal pleistocene early holocene dated remains of extinct mammals for the megafaunal overkill hypothesis

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

hunter gatherer mobility decisions and synchronous climate change in the southern andes the early and middle holocene occupations of arq 18 san juan argentina 29 5 s abstract in the southern andes holocene climate records show drastic changes in moisture during the early and middle holocene to generate a more refined chronology of climate changes in this region we present a bayesian model that combines published cosmogenic dates from the encierro valley 29 1 s and radiocarbon dates on peat and soils from the western slope of the andes 27 33 s we compare this to a similar model from the high altitude archaeological site arq 18 in the las taguas valley 29 5 s san juan argentina these chronologies indicate synchronous changes in climate and occupational intensity which shed light on hunter gatherer mobility decisions this site was first occupied in the early holocene when nearby valleys were deglaciated by around 10 700 cal bp arq 18 was occupied a few centuries later around 10 100 cal bp the site was first colonized during a regional wet phase probably by hunters from the highlands to north who moved quickly among humid high altitude valleys as regional moisture began dropping around 8700 7800 cal bp occupational intensity at arq 18 reached a maximum as diverse groups gathered in the valley at this point an important environmental threshold was crossed as groups reversed their mobility patterns and decisions and did not occupy the site for 1700 years this archaeological silence correlates closely with the middle holocene s hyperaridity during 7800 5700 cal bp as soon as humidity returned groups began visiting the site again from this point on strategies increasingly incorporated herding in response to less stable environmental conditions

archaeology of the pleistocene holocene transition in uruguay an overview abstract in uruguay evidence of early human occupation comes from different archaeological regions including the basin of uruguay and cuareim rivers basin of negro mid river and the atlantic littoral this paper presents a synthesis and overview of the poorly known archaeology of the pleistocene holocene transition of uruguay including archaeological sedimentological and palaeontological information related to the early human occupation in this region the ancient human settlements occurred in various environmental settings including riverbanks hilltops and maritime landscapes new fell type or fishtail projectile points and preliminary results of the current research in pay paso locality are presented evidence from pay paso site 1 indicates that during early holocene the shore of the cuareim river was a paleosurface that was occupied by early human groups a new and previously unpublished type of projectile point from early hunter gatherers ca 9300 9100 yr bp was recovered from excavation of pay paso site 1 finally some comments and observations about culture material and lithic technology for early people of uruguay are explored

mid holocene climatic variability reconstruction from pollen records 32 52 s argentina abstract this study reviews 42 published pollen records to gain a better understanding of holocene climate variability especially of the past displacement and location of the arid semiarid region asr between 32 and 52 s in argentina at present the asr is delimited approximately between the 500 mm isohyet in the pampa region and the 300 mm isohyet in the andean sector south of 37 s represented by arid and semiarid climate types where precipitation can be as low as 200 mm mid holocene climatic variability affected the regional vegetation patterns and the borders of the asr paleoenvironmental reconstruction is based on comparison of fossil pollen records from different depositional environments maps of distribution patterns of plant formations are reconstructed at 2000 500 year intervals from 8000 to 4000 14 c years bp vegetation changes are mainly explained by changes in the position and the seasonal amplitude of latitudinal shifts of the pacific and atlantic anticyclones and of the westerlies from 8000 to 4000 bp the eastern limit of the asr was displaced eastward the western limit reached the maximum eastward displacement in the southern extreme of patagonia at 6000 years bp at this time at the center of asr shrub steppes and the monte developed suggesting maximum dry conditions and the establishment of the arid diagonal north of 42 s while the present day espinal dry pampa ecotone became established at 4000 years bp paleodata suggest a transitional stage towards the present situation of the asr

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

late quaternary vegetation and lake system dynamics in north eastern siberia implications for seasonal climate variability although the climate development over the holocene in the northern hemisphere is well known palaeolimnological climate reconstructions reveal spatiotemporal variability in northern eurasia here we present a multi proxy study from north eastern siberia combining sediment geochemistry and diatom and pollen data from lake sediment cores covering the last 38 000 cal years our results show major changes in pyrite content and fragilarioid diatom species distributions indicating prolonged seasonal lake ice cover between 13 500 and 8900 cal years bp and possibly during the 8200 cal years bp cold event a pollen based climate reconstruction generated a mean july temperature of 17 8 c during the holocene thermal maximum htm between 8900 and 4500 cal years bp naviculoid diatoms appear in the late holocene indicating a shortening of the seasonal ice cover that continues today our results reveal a strong correlation between the applied terrestrial and aquatic indicators and natural seasonal climate dynamics in the holocene planktonic diatoms show a strong response to changes in the lake ecosystem due to recent climate warming in the anthropocene we assess other palaeolimnological studies to infer the spatiotemporal pattern of the htm and affirm that the timing of its onset a difference of up to 3000 years from north to south can be well explained by climatic teleconnections the westerlies brought cold air to this part of siberia until the laurentide ice sheet vanished 7000 years ago the apparent delayed ending of the htm in the central siberian record can be ascribed to the exceedance of ecological thresholds trailing behind increases in winter temperatures and decreases in contrast in insolation between seasons during the mid to late holocene as well as lacking differentiation between summer and winter trends in paleolimnological reconstructions

the recent eutrophication of baldeggersee switzerland as assessed by fossil diatom assemblages diatom analyses with an annual resolution were carried out on varves of the hypertrophic baldeg gersee central swiss plateau for the timespan ad 1885 to 1993 they reveal seven major changes in the dominant planktonic diatoms as a result of progressive nutrient enrichment baldeggersee changed in the 1910s from a cyclotella to a tabellaria fenestrata dominated assemblage and eventually in the 1950s to a stephanodiscus parvusdominated diatom assemblage the timing and direction of diatom assemblage changes in the varved sediment compare well with sedimentological and limnological observations partitioning of the variance in the diatom data revealed that tp is a stronger explanatory variable than temperature for these changes a diatom inferred total phosphorus tp reconstruction indicates three major steps in eutrophication occurring at 1909 the mid 1950s and the mid 1970s comparison with tp measurements in the water column demonstrates that the diatom tp inference model used is able to hindcast past tp concentrations reliably the major steps in eutrophication led to decreases in diatom diversity and also resulted in a progressive increase of calcite grain size the lake restoration programme established since 1982 shows no direct impact on the composition of the diatom assemblages however the decrease in phosphorus loads since the mid 1970s is reflected in the diatom assemblages and in decreasing diatom inferred tp concentrations

a quantitative holocene climatic record from diatoms in northern fennoscandia abstract a diatom based calibration model for predicting summer temperatures was developed using climatically sensitive subarctic lakes in northern fennoscandia the model was applied to a sediment core from a treeline lake to infer trends in holocene climate the record exhibits long term variations as well as a series of shorter term fluctuations on a time scale of centuries summers were warmest in the area about 6200 cal yr b p and featured distinct cooling episodes around 8300 7200 4200 3000 and 400 cal yr b p most of these coinciding with some known climate events e g the 8200 cal yr b p event and the little ice age the similarity of the observed shifts with the pacings of climate events from marine and ice core records represents evidence for coupled ocean atmosphere forcing of the regional climate

pollen distribution in marine surface sediments from chilean patagonia abstract southern south america is the only mid to high latitude continental landmass in the southern hemisphere and therefore represents one of the key areas to recover terrestrial records for the study of past and future climate variability at these latitudes during cruise md 159 of r v marion dufresne cores were taken off the chilean coast from 40 to 55 s in the open ocean and the fjords pollen spectra of 32 core top samples illustrate the relationships between pollen distribution in the sampled marine sediments and their transport from the vegetation onshore pollen concentrations show two distinct gradients 1 a north to south gradient controlled by latitudinal vegetation changes and by westerly winds and 2 from the fjords to the open ocean a gradient clearly controlled by wind fluvial inputs and pollen diagenesis linked to the distance from the coast and water depth the distribution of pollen percentages in these marine surface sediment samples reflects the local vegetation from the nearby continental area consequently pollen analyses from older sediments in these marine cores will be reliable tools to reconstruct past vegetational changes

decadal scale autumn temperature reconstruction back to ad 1580 inferred from the varved sediments of lake silvaplana southeastern swiss alps a quantitative high resolution autumn september november temperature reconstruction for the southeastern swiss alps back to ad 1580 is presented here we used the annually resolved biogenic silic

geomorphology and chronology of late quaternary dune fields of western argentina abstract stabilized dune fields are common in western argentina and potentially reflect drought variability in the late quaternary this study focussed on deciphering geomorphic and stratigraphic record of three vegetated dune fields from east to west san luis medanos negros and medanos grandes with a mean annual precipitation from 700 to 91 mm optically stimulated luminescence single aliquot regeneration protocols yield ages on eolian quartz grains constraining depositional events the san luis dune field the wettest area exhibits evidence for repeated activation during the last glaciation ca 33 20 ka generally coincident with regional loess deposition these dunes were locally reformed in the past 100 years which is apparently coincident with intensified agrarian use and drying in the late 19th and early 20th century the two driest and western most dune fields medanos negros and medanos grandes show reactivation at ca 2 5 ka 0 9 and 0 5 ka and ca 4 3 4 2 1 and 0 6 0 4 ka respectively eolian depositional events at ca 2 5 2 1 ka and 0 6 0 4 ka may be coeval for medanos grandes and negros dune fields sustained drought conditions conducive for eolian activity is associated with warmer sea surface temperatures in the south atlantic a weakened south american convergent zone and a dispersed and southeasterly flow of tropical moisture from the low level chaco jet this nascent analysis indicates drought variability beyond historic observations with potentially at least four dune reactivation events in the past ca 4 5 ka

holocene climatic change in swedish lapland inferred from an oxygen isotope record of lacustrine biogenic silica holocene climatic variability was studied in a 9500 year lake sediment sequence from the abisko region in swedish lapland using the oxygen isotope ratio in diatom biogenic silica d 18 osi oxygen and hydrogen isotope ratios of waters from the abisko area suggest that in this region the evaporative flux is small and the isotopic composition of most lakes reflects that of the local precipitation the hydrological setting of the region and sensitivity analysis of isotopic response to changing climatic parameters such as humidity inflow and evaporation show that the downcore diatom d18osi record is primarily controlled by changes in the summer isotopic composition of the lake water the overall 3 5 depletion in d 18 osi since the early holocene is interpreted as an increase in the influence of the arctic polar continental air mass that carries depleted precipitation we estimate that this change is associated with a 2 5 4 c cooling that has occurred since the early holocene in general the diatom d 18 osi record resembles the average annual air temperature reconstructed for the greenland ice core gisp2 especially during the past 4000 years with a pronounced cooling starting at 2000 years bp

the last glacial maximum climatic conditions on easter island abstract the aim of this paper is to refine previous estimates of last glacial maximum lgm climatic conditions for easter island samples were analysed for fossil pollen from the lower half of a 13 40 m sediment core rra5 this part of the core includes a time span from ca 28 000 10 000 14 c bp the results show that during this period there were three different climatic conditions first a time span from ca 28 000 23 200 14 c bp with slightly cooler and probably drier conditions relative to present day second a more cool and dry climatic condition from ca 23 200 14 700 14 c bp the second time span had two peak points at ca 22 100 and 17 150 14 c bp it seems that the coolest and driest conditions occurred at 17 150 14 c bp third a warm and wet climate started at ca 15 000 14 c bp and continued until the end of the period studied 10 000 14 c bp in all the time covered in this study there was an inverse relationship between values of palmae and poaceae while poaceae and tubuliflorae shrubs show similar variations the temperature estimations that are presented by different authors based on oxygen isotopes are for approximately 4 c cooling at the lgm however the empirical values derived from the pollen evidence suggest only ca 1 9 c cooling this is in agreement with recently obtained values from tropical lowlands

chironomid inferred air temperatures from lateglacial and holocene sites in north west europe progress and problems abstract non biting midges insecta diptera chironomidae are now widely recognised as powerful biological proxies for inferring past climatic change in this paper the response of chironomid assemblages to lateglacial and holocene climatic change from sites in scotland and norway is discussed analyses have shown that midge assemblages have responded to major and minor climatic fluctuations during the last 15 000 yr such as the aegelsee oscillation gerzensee oscillation and younger dryas in the lateglacial and the thermal optimum little ice age and cooling events at 8 2 and 2 0 cal ka bp in the holocene in an attempt to quantify these and other climatic fluctuations a chironomid mean july air temperature inference model based on a modern calibration set of 109 lakes in norway and svalbard has been developed the 3 component weighted averaging partial least squares inference model performs well with a root mean squared error of prediction of 0 90 c and a maximum bias of 0 59 c both based on leave one out cross validation which is lower than chironomid temperature inference models developed in canada switzerland and finland however while temperature inference models perform well in the lateglacial the more subtle climatic changes of the holocene are more difficult to reconstruct because the likely temperature changes are close to the limits of the prediction errors of the existing models to address this problem future work is required to refine and develop the calibration set by further improving taxonomic resolution by adding more lakes to the calibration set and by investigating the influence of other environmental variables on chironomid distributions and abundances

a centennial record of fluvial organic matter input from the discontinuous permafrost catchment of lake tornetrask 1 high latitude regions are underlain by the most organic carbon oc rich soils on earth and currently subject to intense climate warming potentially increasing remobilization and mineralization of soil oc sub arctic scandinavia is located on the 0 c mean annual isotherm and is therefore particularly vulnerable to climate change this study aimed to establish a baseline for soil oc release over the past century into lake tornetrask the largest lake in sub arctic scandinavia through bulk geochemical and molecular radiocarbon analyses in chronologically constrained sediment cores our results suggest a dominance of peat derived terrestrial oc inflow we show that the annual terrestrial oc inflow to the lake is 12 times higher than the in lake produced particulate oc and consists for a large part ca 60 of old oc from deep reservoirs in the catchment the sedimentary record shows signs of increasing inflow of more degraded terrestrial matter since 1975 as indicated by increasing toc concentrations a lower 13c value and lower toc tn ratios based on simultaneous changes in local climate and reported signs of permafrost degradation e g active layer deepening mire peat erosion the observed changes in the sedimentary record of scandinavia s largest mountain lake likely reflect a climate warming induced change in terrestrial oc inflow

holocene millennial scale summer temperature variability inferred from sediment parameters in a non glacial mountain lake danntjorn jotunheimen central southern norway holocene changes in summer temperature in the jotunheimen area have been inferred from variations in several sediment parameters from a composite combined from two overlapping sediment cores lake core record mean time resolution of 15 years from lake danntjorn a small mountain lake located close to the modern pine tree limit in jotunheimen central southern norway loss on ignition analysis at contiguous 0 5 cm intervals throughout the cores revealed that the variations in residue after ignition of the sediments at 5501c in the upper 260 cm 8500 calendar years were not associated with variations in magnetic susceptibility in the basal deglacial and early holocene sequence of the lake sediments however the two sediment parameters fluctuated mainly in phase in addition the biogenic silica bsi concentration is relatively high thus in the early part of the holocene the variations in the residue were probably primarily driven by input of minerogenic detrital material inwash and or windblown and biological productivity including bsi from diatoms in the lake itself and in the lake surroundings from approximately 8500 calendar years bp variations in the residue probably primarily reflect variations in low minerogenic low magnetic susceptibility with little variations and diatom bsi productivity as demonstrated with analysis of bsi concentration between 260 and 233 cm 8500 7760 calendar years bp the highest residue values probably mainly reflecting high diatom productivity occurred at 7800 7200 7000 6700 6200 5400 5050 4300 2500 1800 1550 900 700 and o100 calendar years bp the most significant o8000 calendar years bp residue minima mainly low diatom productivity were recorded at 7100 6800 6600 6000 5700 5000 4400 4200 3800 3600 3200 3000 2750 2300 1900 1700 1070 740 600 380 and 80 calendar years bp bp ad 2000 the lake record from jotunheimen is discussed in the context of holocene terrestrial and marine records from the north atlantic region and possible natural climate forcing factors r 2004 elsevier ltd all rights reserved

diatom response to mid holocene climate in three small arctic lakes in northernmost finnmark palaeoclimatic reconstructions from lake sediment biological records can be challenging due to variation in non climatic factors which alter ecosystem responses to consider this it is important to replicate a study regionally so as to gain information on spatial variability of ecosystem response and the influence of site specific conditions previous pollen based palaeoclimatic records from three well dated arctic lake sites highlight the response of regional scots pine pinus sylvestris and mountain birch betula pubescens ssp czerepanovii forest tundra transition to holocene climatic variability and suggest the northernmost peninsulas of finnmark to be climatically sensitive this study analysed dated sediment sequences between c 3970 and c 6200 cal yr bp from these three previously published shallow lakes liten cap pesjav ri lcj over gunnarsfjorden ogf and over kobbkrokvatnet okv for freshwater diatoms diatom assemblages showed an increase in the planktonic diatoms relative to benthic diatoms with an onset towards higher abundances of small centric planktonic diatoms at ogf between c 5270 and 5350 cal yr bp and okv between c 5280 and 5350 cal yr bp additionally a diatom compositional shift was detected at lcj between c 5180 and 5300 cal yr bp trend analysis found the main diatom compositional change at all three sites to occur at 5300 cal yr bp however this synchronous diatom shift during the mid holocene varies in magnitude within the three finnmark lakes the abrupt planktonic benthic diatom ratio changes are independent of the pinus betula ratios and are likely to be a result of lake ice cover changes allowing longer growing seasons greater water column stability and higher nutrient concentrations from surrounding catchments this study highlights the significant differences in aquatic ecosystem and terrestrial vegetation response to climatic changes with diatom assemblages at these sites either responding before the regional tree line shift or to a separate climatic event within the mid holocene

environmental variability in northeastern siberia during the last 13 300 yr inferred from lake diatoms and sediment geochemical parameters abstract in northeastern siberia russia a 1 2 m sediment core was retrieved and radiocarbon dated from a small and shallow lake located at the western side of the lower lena river n 69 24 e 123 50 81 m a s l the objective of this paper is to reconstruct the palaeoenvironmental variability and to infer major palaeoclimate trends that have occurred since 13 3 cal kyr bp we analysed the diatom assemblages sedimentology grain size total organic carbon toc total nitrogen tn and the elemental and mineralogical composition using x ray fluorescence xrf and x ray diffractometry xrd of the sediment core our results show parallel changes in the diatom species composition and sediment characteristics enhanced minerogenic sediment input and the occurrence of pyrite is indicative of a cold period between 12 7 and 11 6 cal kyr bp the diatom data enable a qualitative inference about the local ecological conditions to be made and reveal an oligotrophic lake system with alkaline and cold conditions during the earliest holocene moderately warmer climates are inferred for the period from 9 1 to 5 7 cal kyr bp the major shift in the diatom assemblage from dominance of small benthic fragilarioid taxa to a more complex diatom flora with an influx of several achnanthoid and naviculoid diatom species occurred after a transitional period of about 1400 yr 7 1 to 5 7 cal kyr bp at 5 7 cal kyr bp indicating a circumneutral and warmer hydrological regime during the holocene thermal maximum htm diatom valve concentrations declined starting 2 8 cal kyr bp but have been rising again since 600 cal yr bp this has occurred in parallel to the increased presence of acidophilous diatom taxa e g eunotia spp and decreased presence of small benthic fragilarioid species in the most recent sediments which is interpreted as the result of neoglacial cooling and subsequent recent climate warming our findings are compared to other lake inferred climate reconstructions along the lena river we conclude that the timing and spatial variability of the htm in the lower lena river area reveal a temporal delay from north to south

active earth pressure for soils with tension cracks under steady unsaturated flow conditions the calculation of active earth pressure has commonly been performed under dry or saturated conditions however in practice soils are usually unsaturated aiming at the estimation of active earth pressure under steady unsaturated flow conditions this paper describes a new method within the framework of the kinematic approach a closed form formula is adopted to estimate the suction stress of unsaturated soils subjected to vertical steady flow a rotational failure mechanism composed of a log spiral curve and a vertical crack is presented for calculations of the external work rate and internal energy dissipation rate pre existing cracks as well as cracks that form as part of the failure mechanism are considered in the present work the maximum depth of cracks is determined by requiring the vertical crack boundary to be stable explicit expressions about the thrust of active earth pressure for different crack types are derived from the work energy balance equation and the most critical solutions are obt

complex pedogenesis related to differential aeolian sedimentation in microenvironments of the southern part of the semiarid region of argentina abstract the aim of this paper is to explain soil genesis and spatial variability in the soil cover of a flat landscape in the southern part of argentina s semiarid region soil survey data indicate random differences in the properties of soils lying within a few hundred meters of one another as reflected in an intricate distribution pattern which cannot be explained by the climatic theory of soil genesis in a single pedogenetic cycle this pattern is unrelated to the actual vegetation cover the soil parent materials consist of a

diatom community response to multiple scales of holocene climate variability in a small lake on victoria island nwt canada abstract a lake sediment core spanning 9900 years collected from a small lake on western victoria island nwt canada provides a high resolution record of diatom community dynamics over the holocene ten radiocarbon dates and 210 pb dating provided the core chronology loss on ignition loi magnetic susceptibility biogenic silica content and diatom concentrations provided information on changes in the sedimentary environment loi gradually increased over the holocene whereas magnetic susceptibility showed an inverse trend biogenic silica content showed three distinct peaks spaced approximately 3000 years apart major shifts in diatom assemblages occurred at 8100 8000 5800 5700 and 3800 3500 cal yr bp there is evidence of diatom community response to centennial scale variations such as the medieval warm period 1000 700 cal yr bp little ice age 800 150 cal yr bp and recent warming although recent changes in diatom community composition productivity and species richness are apparent they were surpassed at other periods throughout the holocene variations of the taxa within the genera staurosira pseudostaurosira fragilaria and staurosirella usually combined into one genus in arctic lake sediment studies suggest these taxa provide useful insight into paleonvironmental questions

polar perspective of late quaternary climates in the southern hemisphere abstract late quaternary paleoecological and glacial evidence from the higher latitudes of the southern hemisphere implies overall uniformity of large scale glacial and interglacial climatic fluctuations for the past 40 000 yr climate of the last glacial maximum variously dated between 30 000 and 11 000 yr b p was relatively cold and dry compared with the warmer more humid climate of the holocene and the interstade preceding the last glacial maximum conditions were apparently coldest during millennia centered around 20 000 yr b p and warmest in the early holocene recorded small scale fluctuations frequently variable for any given time interval are less consistent a cold late glacial episode estimated as occurring between ca 13 000 and 11 000 yr b p in antarctica possibly was coeval with the younger dryas stade in northwestern europe and may be correlative with a climatic episode in southern south america and perhaps in new zealand and south georgia however there is no evidence for the event in tasmania general atmospheric circulation models for the polar latitudes at the time of the last glacial maximum show an intensification of the southern westerlies apparently a result of the expansion of ice cover in antarctica and of sea ice in the southern ocean

diatoms as quantitative indicators of july temperature a validation attempt at century scale with meteorological data from northern sweden diatoms as quantitative indicators of july temperature a century scale validation with meteorological data from northern sweden

late glacial holocene climate of the lake district of chile pollen evidence from a 350 cm section of a fen in a moraine belt at rucanancu 39 33 s 72 18 w bears on the controversy regarding interpretation of late glacial and holocene climate in midlatitude chile earlier pollen studies indicating a cooling trend between approximately 11 000 and 10 000 yr b p disagreed with observations of glacier fluctuations which show continuous glacier wastage and by inference warming after 12 500 yr b p and possibly earlier up until neoglaciation beginning after 6850 yr b p fossil beetle assemblage data in this time range support the interpretation of climate made from the observed glacier behavior at rucanancu a pollen assemblage containing upper montane podocarp podocarpus andinus in quantities reaching 34 and dating between 10 440 and 10 000 yr b p implies a cold climate with summer temperatures possibly 5 8 c lower than today s holocene warming began afterward later than the glacier and beetle records indicate and continued until at least 8350 yr b p as suggested by the sequence of assemblages dominated by myrtaceae by aextoxicon punctatum and by gramineae a subsequent assemblage of nothofagus obliqua type implies an increase of moisture until 6960 yr b p following which n dombeyi type under a cool and humid neoglacial climate became dominant

palynology of cushion bogs of the cordillera pelada province of valdivia chile abstract fossil pollen identified in the earliest sediments of three cushion bogs in the cordillera pelada 40 10 s 73 30 w dated 10 425 14 c yr b p includes the subantarctic species dacrydium fonckii tetroncium magellanicum astelia pumila gaimardia australis donatia fascicularis and drosera uniflora all grow today in the cordillera pelada and range poleward to the southernmost province of magallanes one species drapetes muscosa included with the pollen of these plants in the earliest record is no longer a constituent of the flora but is limited only to subantarctic chile available evidence indicates that plants survived the last glaciation north of the glacial border with the course of postglacial migration southward following the wastage of the glacier complex holocene climatic and vegetational changes in the cordillera pelada are interpreted in the context of regional reconstructions which show maximum warmth about 9000 yr ago with a pronounced dry period lasting from 9000 to 6500 yr b p maximum precipitation was later reached around 4000 yr ago but has decreased overall since then the regional decline of the endemic gymnosperm fitzroya cupressoides which today is extensively destroyed in the cordillera pelada follows this decrease in precipitation these climatic data suggest a net south ward shift in the zone of westerly winds that bring rainfall to the region over the past 4000 yr

latest pleistocene and holocene alpine glacier fluctuations in scandinavia abstract during the early holocene abrupt decadal to centennial scale climate variations caused significant glacier variations in norway increased freshwater inflow to the north atlantic and arctic oceans has been suggested as one of the most likely mechanisms to explain the abrupt and significant lateglacial and early holocene climatic events in nw europe the largest early holocene glacier readvances occurred 11 200 10 500 10 100 9700 9200 and 8400 8000 cal yr bp the studied norwegian glaciers apparently melted away at least once during the early mid holocene the period with the most contracted glaciers in scandinavia was between 6600 and 6000 cal yr bp subsequent to 6000 cal yr bp the glaciers started to advance and the most extensive glaciers existed at about 5600 4400 3300 2300 1600 cal yr bp and during the little ice age times with overall less glacier activity were apparently around 5000 4000 3000 2000 and 1200 cal yr bp it has been proposed that several glacier advances occurred in scandinavia including northern sweden at 8500 7900 7400 7200 6300 6100 5900 5800 5600 5300 5100 4800 4600 4200 3400 3200 3000 2800 2700 2000 1900 1600 1200 1000 and 700 200 cal yr bp glaciers in northern sweden probably reached their greatest little ice age extent between the 17th and the beginning of the 18th centuries evidence for early holocene glacier advances in northern scandinavia however has been questioned by more recent multi disciplinary studies the early to mid holocene glacier episodes in northern sweden may therefore be questioned most norwegian glaciers attained their maximum little ice age extent during the mid 18th century cumulative glacier length variations in southern norway based on marginal moraines dated by lichenometry and historic evidence show an overall retreat from the mid 18th century until the 1930s 40s subsequently most norwegian glaciers retreated significantly maritime outlet glaciers with short frontal time lags 15 20 years continued their retreat to the 1970s and 1980s however maritime glaciers started to advance as a response to higher winter accumulation during the first part of the 1990s after 2000 several of the observed glaciers have retreated remarkably fast annual frontal retreat 100 m mainly due to high summer temperatures the general glacier retreat during the early holocene and the neoglacial advances after 6000 cal yr bp are in line with orbital forcing due to the decrease of northern hemisphere summer solar insolation and the increase in winter insolation in addition regional weather modes such as the north atlantic oscillation nao and the arctic oscillation ao play a significant role with respect to decadal and multi decadal climate variability

historical trends in lake and river ice cover in the northern hemisphere freeze and breakup dates of ice on lakes and rivers provide consistent evidence of later freezing and earlier breakup around the northern hemisphere from 1846 to 1995 over these 150 years changes in freeze dates averaged 5 8 days per 100 years later and changes in breakup dates averaged 6 5 days per 100 years earlier these translate to increasing air temperatures of about 1 2 c per 100 years interannual variability in both freeze and breakup dates has increased since 1950 a few longer time series reveal reduced ice cover a warming trend beginning as early as the 16th century with increasing rates of change after about 1850

hot or not impact of seasonally variable soil carbonate formation on paleotemperature and o isotope records from clumped isotope thermometry quantifying the timing and temperature of soil carbonate formation is important for interpreting isotopic records of earth surface temperature and soil water composition from paleosols pedogenic carbonates typically have been assumed to form at mean annual temperature but recent work suggests warm season bias in carbonate formation could impact the interpretation of d 18 o values and carbonate

major hydrological regime change along the semiarid western coast of south america during the early holocene water availability in the semiarid western coast of chile 30 32 s is conditioned by high interannual precipitation variability reflecting the transition between arid subtropical and moist mid latitude climates in the southeastern pacific ocean a paleoclimate reconstruction based on the latest pleistocene holocene geological record from the quebrada santa julia archeological site in chile 31 50 s and on modern meteorological mechanisms producing alluvial episodes in this region indicates a major change in the rainfall regime shortly after 8600 cal yr bp this together with other paleoclimate proxies along the west coast of south america 34 14 s suggests la nina like conditions 13 000 8600 cal yr bp based on sedimentological and geomorphologic evidence we hypothesized that the absence of heavy rainfall events in northern chile and the new hydrological regime that prevailed ca 8600 5700 cal yr bp in north central chile resulted from an increase in the large scale westerly flow over central chile as expected in near neutral enso conditions this atmospheric circulation anomaly is compatible with an equatorward shift of the influence of the southeast pacific subtropical anticyclone relative to the early holocene prior to the onset of modern enso variability

large rockslides in the southern central andes of chile 32 34 5 s tectonic control and significance for quaternary landscape evolution abstract the distribution and age of large 0 1 km2 pliocene to recent rockslides in the chilean cordillera principal 32 34 5 s the southern central andes has been analyzed to determine the rockslide triggering mechanisms and impact on regional landscape evolution most of the rockslides appear in the western cordillera principal and cluster along major geological structures variographic analyses show spatial correlation between rockslides geological structures and shallow seismicity a relative chronosequence was calibrated with existing 14c and 40ar 39ar dates and new cosmogenic nuclide exposure ages for selected rockslides rockslide induced sediment yield was estimated with empirical relations for rockslide area distributions throughout the quaternary rockslides have delivered sediment to streams at rates equivalent to denudation rates of 0 10 0 06 mm a 1 while estimates using short term 20 a seismicity records are 0 3 0 2 0 6 mm a 1 the estimates of sediment transfer and the spatial distribution of rockslides reflect a landscape in which tectonic and geological controls on denudation are more significant than climate

late quaternary vegetation and climate history of a perennial river canyon in the rio salado basin 22 s of northern chile abstract plant macrofossils from 33 rodent middens sampled at three sites between 2910 and 3150 m elevation in the main canyon of the rio salado northern chile yield a unique record of vegetation and climate over the past 22 000 cal yr bp presence of low elevation prepuna taxa throughout the record suggests that mean annual temperature never cooled by more than 5 c and may have been near modern at 16 270 cal yr bp displacements in the lower limits of andean steppe and puna taxa indicate that mean annual rainfall was twice modern at 17 520 16 270 cal yr bp this pluvial event coincides with infilling of paleolake tauca on the bolivian altiplano increased enso activity inferred from a marine core near lima abrupt deglaciation in southern chile and heinrich event 1 moderate to large increases in precipitation also occurred at 11 770 9550 central atacama pluvial event 7330 6720 3490 2320 and at 800 cal yr bp desiccation occurred at 14 180 8910 8640 and 4865 cal yr bp compared to other midden sites in the region early holocene desiccation seems to have happened progressively earlier farther south emerging trends from the cumulative midden record in the central atacama agree at millennial timescales with improved paleolake chronologies for the bolivian altiplano implying common forcing through changes in equatorial pacific sea surface temperature gradients

chilean and southeast pacific paleoclimate variations during the last glacial cycle directly correlated pollen and δ18o records from odp site 1234 abstract joint pollen and oxygen isotope data from ocean drilling program site 1234 in the southeast pacific provide the first continuous record of temperate south american vegetation and climate from the last 140 ka located at 36 s 65 km offshore of concepcion chile site 1234 monitors the climatic transition zone between northern semi arid summer dry winter wet climate and southern year round rainy cool temperate climate dominance of onshore winds suggests that pollen preserved here reflects transport to the ocean via rivers that drain the region and integrate conditions from the coastal mountains to the andean foothills down hole changes in diagnostic pollen assemblages from xeric lowland deciduous forest characterized by grasses herbs ferns and trees such as deciduous beech nothofagus obliqua mesic valdivian evergreen forest including conifers such as the endangered prumnopitys andina and subantarctic evergreen rainforest comprised primarily of southern beech n dombeyi reveal large rapid shifts that likely reflect latitudinal movements in atmospheric circulation and storm tracks associated with the southern westerly winds during glacial intervals mis 2 4 and 6 rainforests and parkland dominated by nothofagus moved northward into the region at the mis 6 5e transition coeval with the rapid shift to lower isotopic values rainforest vegetation was rapidly replaced by xeric plant communities associated with mediterranean type climate an increased prominence of halophytic vegetation suggests that mis 5e was more arid and possibly warmer than mis 1 although rainforest pollen rises again at the end of mis 5e lowland deciduous forest pollen persists through mis 5d and 5c into mis 5b substantial millennial scale variations occur in both interglacial and glacial regimes attesting to the sensitivity of the southern westerly belt to climate change comparison of the cool mesic n dombeyi rainforest assemblage from site 1234 with 18 o in the byrd ice core shows that on time scales longer than 10 ka cool moist conditions in central chile were coherent with and occurred in phase with antarctic cooling this is also likely at millennial scales although rainforest pollen lags antarctic cooling with exponential response times of about 1000 years which plausibly reflects the ecological response time to regional climate change

a coulomb type solution for active earth thrust with seepage a coulomb type general solution for active earth pressure on the vertical face of a retaining wall with a drainage system along the soil structure interface is presented the soil is cohesionless and fully saturated to the ground horizontal surface this condition may happen during heavy rainfall and is the most critical when the active pressure reaches its peak value in order to solve the problem a theoretical closed form solution for the water seepage through the soil is first developed this is used in a coulomb type formulation which supposes a plane failure surface inside the backfill when the wall movement is enough to put the soil mass in the active state the formulation provides coulomb like coefficients of active pressure which solve the problem in a generic way comparison with published results obtained by hand calculation shows very good agreement a table with values of the coefficients of active earth pressure with seepage calculated for selected values of the soil internal friction a

timing and nature of alluvial fan and strath terrace formation in the eastern precordillera of argentina abstract sixty eight 10 be terrestrial cosmogenic nuclide tcn surface exposure ages are presented to define the timing of alluvial fan and strath terrace formation in the hyper arid san juan region of the argentine precordillera this region is tectonically active and numerous fault scarps traverse quaternary landforms the three study sites marquesado strath complex loma negra alluvial fan and carpinteria strath complex reveal a history of alluvial fan and strath terrace development over the past 225 ka the marquesado complex q3 m surface dates to 17 3 ka whereas the loma negra q1 ln q2 ln q3 ln q4 ln and q5 ln surfaces date to 24 3 ka 48 2 ka 65 13 ka 105 21 ka and 181 29 ka respectively the carpinteria complex comprises eight surfaces that have been dated and include the q1 c 23 3 ka q2 c 5 5 ka q3a c 25 12 ka q3b c 29 15 ka q4 c 61 12 ka q5 c 98 18 ka q6 c 93 18 ka and q7 c 212 37 ka 10 be tcn depth profile data for the loma negra alluvial fan complex and carpinteria strath terrace complex as well as osl ages on some carpinteria deposits aid in refining surface ages for comparison with local and global climate proxies and additionally offer insights into inheritance and erosion rate values for tcns 10 10 4 10 be atoms g of sio 2 and 5 m ma 1 respectively comparison with other alluvial fan studies in the region show that less dynamic and older preserved surfaces occur in the carpinteria and loma negra areas with only younger alluvial fan surfaces preserved both to the north and south these data in combination with that of other studies illustrate broad regional agreement between alluvial fan and strath terrace ages which suggests that climate is the dominant forcing agent in the timing of terrace formation in this region

the pleistocene holocene transition and human occupation in the central andes of argentina agua de la cueva locality in the central andes of argentina evidences of early human occupations come from the precordillera a low mountain system which was under periglacial conditions prior to 14 000 14c yr bp when the cordillera principal and cordillera frontal were glaciated the corridors for human migration across the andes may have opened before 13 000 14c yr bp the pleistocene holocene transition gave rise to a substantial variability of environments and consequently of natural resources for early people who found in the precordillera new ecosystems with suitable conditions for living the first human groups arrived ca 11 000 14c yr bp at agua de la cueva rockshelter located at 2900 m above sea level in the western flank of the precordillera the occurrence of raw material outcrops close to the site and the archaeofaunistic record indicate a local and likely seasonal exploitation of mountain resources these early people lived in a shrub steppe environment under cooler and probably wetter conditions in which camelids were the most important food resource since 9000 14c yr bp warmer and drier conditions were dominant except for a lower intensity of human occupations the site function of agua de la cueva seems to have remained the same

quaternary glaciations in the rio mendoza valley argentine andes in the rio mendoza valley five pleistocene drifts and one holocene drift are distinguished by multiple relative age criteria including surface rock weathering development of rock varnish moraine morphology soil profile development and stratigraphic relationships several absolute ages suggest a preliminary chronology during the oldest uspallata glaciation a system of valley glaciers flowed 110 km from the andean drainage divide and 80 km from cerro aconcagua to terminate at 1850 m drift of this ice advance is older than a widespread tephra dated by fission track at 360 000 36 000 yr during the punta de vacas advance ice terminated at 2350 m while during the subsequent penitentes advance the glacier system ended at 2500 m a travertine layer overlying penitentes drift has u series age of 24 200 2000 yr b p the distribution of horcones drift which is inferred to represent the last glacial maximum delimits an independent ice stream that flowed 22 km down horcones valley to 2750 m a later readvance almacenes reached 3250 m confluencia drift is considered to be neoglacial in age and extends downvalley to 3300 m the moraine sequence is compared with those studied by caviedes 1972 along rio aconcagua on the chilean flank of the andes

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

ice age vegetation and climate of subtropical chile abstract vegetation and climate of subtropical chile over the past 50 000 yr are reconstructed from pollen spores and other microfossils contained in a 10 7 m section of lacustrine deposits of laguna de tagua tagua 34 30 s 71 10 w controlled by 14 radiocarbon age determinations five pollen assemblages covering stratigraphic zones in the section represent plants on the upland and species inhabiting the lake basin surface pollen spectra from 58 stations approximately 30 40 s form an adjunct for interpreting past vegetation and climate from the fossil record temperature semi humid woodland of southern beech nothofagus dombeyi type and podocarp prumnopitys andina was apparently established about laguna de tagua tagua during the pleistocene contrasting existing semi arid broad sclerophyllous vegetation the woodland was extensive about the lake at the time of the last glacial maximum 25 000 14 000 yr b p and earlier 43 000 34 000 yr b p pollen from the families chenopodiaceae and amaranthaceae which characterize intervals of the pleistocene centered around 33 000 and 43 000 yr b p and the holocene suggests intervening episodes of relative aridity lake level fluctuations appear to follow the late quaternary climatic pattern implied by pollen assemblage data water in the lake was relatively high during development of southern beech podocarp woodland whereas during intervals of chenopod amaranth dominance lake levels were comparatively low excess precipitation over evaporation which during the pleistocene favored development of woodland of beech and podocarp is attributed to greater storm frequency conditions were apparently governed by strengthening of atmospheric circulation in the belt of southern westerly winds in the late glacial the temperate and more humid ice age climate with limited seasonality underwent transition to a subtropical semi arid mediterranean type that identifies the postglacial fluctuations in intensity of the southern westerlies marked by latitudinal movement of the polar front can account for past vegetation changes at laguna de tagua tagua

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

palaeohydrology of laguna de tagua tagua 34 30 s and moisture fluctuations in central chile for the last 46 000 yr central chile 32 35 s lies at the northern border of the strong influence of the westerly circulation belt and thus exhibits a steep rainfall gradient a new core from laguna de tagua tagua 34 30 0 s provides a sedimentologic geochemical and palynological record of regional hydrologic balance for the last 46 000 cal yr bp according to our age model relatively humid conditions occurred during glacial times before 43 500 cal yr bp and from 40 000 to 21 500 cal yr bp reduced moisture conditions and likely lower temperatures occurred from 42 400 40 100 cal yr bp higher lake levels and pollen assemblages with valdivian rainforest taxa imply much higher precipitation during glacial times 40 100 21 000 cal yr bp compared to today and therefore enhanced westerly activity in northern central chile afterwards the general decrease in moisture was punctuated by two abrupt arid periods at 21 000 19 500 cal yr bp and 17 000 15 000 cal yr bp and two more humid intervals 19 500 17 000 almost coincident with the global last glacial maximum lgm and 13 500 11 500 cal yr bp the early and mid holocene were the most arid periods in central chile for the studied time interval millennial scale palaeohydrological reconstructions from tagua tagua are consistent with regional climatic records copyright 2005 john wiley sons ltd

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

expansion of magellanic moorland during the late pleistocene palynological evidence from northern isla de chiloe chile the late quaternary vegetation of northern isla de chiloe is inferred from palynological analysis of a section in the rio negro drainage 42 03 s 73 50 w at ca 30 500 yr b p maxima of astelia and donatia occurred suggesting wetland development from that time until ca 27 000 yr b p steppe indicators such as compositae gramineae dominated suggesting drier conditions after 27 000 yr b p the moorland shrub dacrydium gradually increased reaching a maximum by 18 000 yr b p at this time astelia increased again suggesting development of cushion bog during cold and wet conditions the glacial postglacial transition is characterized by a marked change from peaty sediments to clays a decrease in the cushion bog flora and the prevalence of gramineae compositae and swamp taxa this vegetation prevailed until ca 7000 yr b p when forest taxa became dominant the floristic pattern inferred from the pollen spectra of the rio negro section suggests that the late pleistocene vegetation of chiloe resembled modern magellanic moorland vegetation 52 56 lat s based on climatic conditions presently associated with magellanic moorland its occurrence in chiloe at low elevations during the late pleistocene implies a decrease in average temperature of at least 4 c and an increase in annual precipitation of at least 1500 mm

coefficients of active earth pressure with seepage effect a calculation method for the active earth pressure on the possibly inclined face of a retaining wall provided with a drainage system along the soil structure interface is presented the soil is cohesionless and fully saturated to the ground surface this situation may arise during heavy rainstorms to solve the problem the water seepage through the soil is first analyzed using a numerical procedure based on the boundary element method then the obtained pore water pressure is used in a coulomb type formulation which supposes a plane failure surface inside the backfill when the wall movement is enough to put the soil mass in the active state the formulation provides coefficients of active pressure with seepage effect which can be used to evaluate the active earth thrust on walls of any height a series of charts with values of the coefficients of active earth pressure with seepage calculated for selected values of the soil internal friction angle the wall soil friction angle and the wall face inclina

a review of tertiary climate changes in southern south america and the antarctic peninsula part 2 continental conditions abstract climate changes in southern south america and the antarctic peninsula during the tertiary show a strong correlation with ocean warming and cooling events which are in turn related to tectonic processes during periods of accelerated sea floor spreading and mid ocean ridge activity sea levels rose so that parts of the continents were flooded and forests were destroyed however this was balanced by the large scale release of co2 during volcanic outgassing and carbonate precipitation on the continental shelves which caused rising air temperatures and the poleward expansion of sub tropical and temperate forests cooling episodes generally caused an increase in the north south thermal gradient because of an equatorward shift in climate belts so that the westerly winds intensified and brought higher rainfall to the lower latitudes an increase in wind blown dust caused temperatures to drop further by reflecting sunlight back into space the rising andes range had a marked influence on climate patterns up to the middle miocene it was still low enough to allow summer rainfall to reach central and north central chile but after about 14 ma it rose rapidly and effectively blocked the spill over of moisture from the atlantic ocean and amazon basin at this time the cold humboldt current was also established which together with the andes helped to create the arid diagonal of southern south america stretching from the atacama desert to the dry steppes of patagonia this caused the withdrawal of subtropical forests to south central chile and the expansion of sclerophytic vegetation to central chile however at the same time it intercepted more rain from the northeast causing the effect of the south american monsoon to intensify in northwestern argentina and southern bolivia where forest communities presently occur in patagonia glaciation started as early as 10 5 ma but by 7 ma had become a prominent feature of the landscape and continued apparently uninterruptedly into the pleistocene the antarctic peninsula saw its first mountain glaciation between 45 and 41 ma with major ice sheet expansion commencing at about 34 ma isolated stands of nothofagus forests were still present in low lying areas suggesting that the glaciers were initially wet based but dry based glaciers were established at around 8 ma although temperatures rose briefly during the messinian pliocene transition causing sub antarctic flora to retreat to higher elevations of the transantarctic mountains the present cold polar conditions were finally established by about 3 ma

similarities and discrepancies between chironomid and diatom inferred temperature reconstructions through the holocene at lake 850 northern sweden similarities and discrepancies between chironomid and diatom inferred temperature reconstructions through the holocene at lake 850 northern sweden

diatom response to climate forcing of a deep alpine lake lugu hu yunnan sw china during the last glacial maximum and its implications for understanding regional monsoon variability abstract the yunnan plateau of southwest china is strongly influenced by the asian monsoon and is a critical area in terms of determining its development and variability since the last glacial maximum high resolution diatom analysis of a 14c dated sediment core retrieved from lugu lake yunnan provides a detailed history of palaeoenvironment changes from 30 000 to 10 000 cal yr bp a model of climate diatom interactions linking thermal stratification ice cover and catchment derived nutrients is proposed to explain the observed changes prior to 24 500 cal yr bp the dominance of the planktonic cyclotella rhomboideo elliptica indicates a period of stable stratification and low nutrient levels with a moderate change of temperature and precipitation in the southwest monsoon region during the lgm 24 500 14 500 cal yr bp the loss of the planktonic diatoms and a switch to small fragilaria spp suggest an altered thermal regime in the lake i e weaker stratification and possibly ice cover from 18 000 cal yr bp the expansion of planktonic taxa stephanodiscus and the asterionella formosa and increased toc are indicative of increased nutrient availability reflecting altered vegetation and soil processes due to rising regional temperatures after 14 500 cal yr bp a rapid reversal to dominance by planktonic diatoms indicates a further increase in temperature and continued development of catchment vegetation and soils the abrupt changes in diatom species composition around 24 500 and 14 500 cal yr bp suggest that the ecological status of the lake exceeded limnological thresholds interestingly however the younger dryas event is not clearly recognizable in either the diatom or pollen records in the lugu lake sediment record in a partial redundancy analysis 44 2 of the variability in the total diatom assemblages in the period 30 000 10 000 cal yr bp was accounted for by changes in summer solar radiation the relationships between the diatom assemblages and the c n and pollen data are further evidence of climate catchment vegetation interactions and their impact on lake functioning the composition of diatom assemblages in the lugu lake sediment core reflects direct and indirect i e via catchment processes climate forcing and in particular the strength of the southwest monsoon which in turn corresponds to orbitally induced variability in northern hemisphere summer solar insolation from 30 000 cal yr bp to 10 000 cal yr bp

marine radiocarbon reservoir effect along the northern chile southern peru coast 14 24 s throughout the holocene abstract through an extensive sampling and dating of pairs of associated shells and charcoal fragments combined with reanalysis of all the available previous data we reconstruct the evolution throughout the holocene of the regional marine radiocarbon reservoir effect r values along the northern chile southern peru area 14 24 s after elimination of the cases in which the terrestrial component yielded older ages than the marine shells to which they were associated the study is based upon data from 47 pairs of associated marine and terrestrial material our results suggest major changes in both the magnitude and variability range of r during the whole holocene period 1 between 10 400 and 6840 cal yr bp high values 511 278 yr probably result from a strengthened se pacific subtropical anticyclone and shoaling of equatorial subsurface waters during intensified upwelling events 2 between 5180 and 1160 cal yr bp lower values 226 98 yr may reflect a major influence of subtropical water and diminished coastal upwelling processes 3 during the last thousand years high values between 355 105 and 253 207 yr indicate an increased influence of 14 c depleted water masses and of enso for the early twentieth century a r value of 253 207 yr was calculated