late glacial and holocene vegetational history from two sites in the western cordillera of southwestern ecuador the deciphering of the late glacial and holocene vegetation glacial geological and climatic history of cajas national park in southwestern ecuador is undertaken focusing on close interval sampling of sediment cores from two high elevations lakes lagunas chorreras 3700 m and pallcacocha 4060 m this study involves extensive dating both accelerator mass spectrometry and tephra palynological and sediment analyses of lakes and bogs basal dates for the two cores analyzed for pollen range from about 17 000 to about 15 500 cal yr bp vegetation surveys and surface sample pollen and spore analyses were accomplished for two transects in the western cordillera one began east of the main divide near cuenca ecuador and the other began at the crest of the western cordillera descending towards the pacific ocean these vegetation surveys coupled with pollen analyses of surface samples were used to establish pollen analogues to help in the interpretation of the fossil pollen records the results of the pollen analyses from the two lake cores indicate two major climatic periods 1 the late glacial pollen record beginning about 17 000 yr bp and ending near 11 000 is characterized by an herb paramo dominated by pteridophytes primarily huperzia spp with asteraceae assumed to be woody species and puya spp the climate inferred from these pollen records was cooler and moister than today the sediments for this time period are characterized by low loss on ignition percentages and high magnetic susceptibility values with secondary peaks between 12 000 and 13 000 yr bp low concentrations of charcoal fragments indicate a low incidence of fire and several pronounced fluctuations in pollen assemblages could be interpreted as changes in the prevailing wind direction and or climate 2 the beginning of the holocene as represented in the pollen record is characterized by the disappearance of puya pollen vastly diminished huperzia representation high charcoal concentrations and the expansion of moist montane forest pollen asteraceae possibly gynoxys are replaced as dominant timberline taxa by polylepis dominated timberline forest the charcoal record suggests that fires were much more prevalent during the early to middle holocene than during the late glacial changes in isoe tes concentrations and trends in upland vegetation may be related to fluctuations in lake levels and precipitation

15 000 yr pollen record of vegetation change in the high altitude tropical andes at laguna verde alta venezuela pollen analysis of sediments from a high altitude 4215 m neotropical 9 n andean lake was conducted in order to reconstruct local and regional vegetation dynamics since deglaciation although deglaciation commenced 15 500 cal yr b p the area around the laguna verde alta lva remained a periglacial desert practically unvegetated until about 11 000 cal yr b p at this time a lycopod assemblage bearing no modern analog colonized the superparamo although this community persisted until 6000 cal yr b p it began to decline somewhat earlier in synchrony with cooling following the holocene thermal maximum of the northern hemisphere at this time the pioneer assemblage was replaced by a low diversity superparamo community that became established 9000 cal yr b p this replacement coincides with regional declines in temperature and or available moisture modern more diverse superparamo assemblages were not established until 4600 cal yr b p and were accompanied by a dramatic decline in alnus probably the result of factors associated with climate humans or both pollen influx from upper andean forests is remarkably higher than expected during the late glacial and early to middle holocene especially between 14 000 and 12 600 cal yr b p when unparalleled high values are recorded we propose that intensification of upslope orographic winds transported lower elevation forest pollen to the superparamo causing the apparent increase in tree pollen at high altitude the association between increased forest pollen and summer insolation at this time suggests a causal link however further work is needed to clarify this relationship

biological responses to rapid climate change at the younger dryas holocene transition at krakenes western norway a fine resolution pollen stratigraphical study supported by macrofossil analysis has been made at krakenes lake western norway through the younger dryas holocene transition and the early holocen

evidence for climate instability during the younger dryas interval in the hengduan mountains yunnan southwestern china abstract the younger dryas yd is the most recent abrupt climatic event recognized during the transition from the last glaciation to the holocene growing evidence from the middle and high latitudes of europe and north america shows that the climate within the yd event was highly unstable until recently however climate instability during this event has received little attention elsewhere in the world here we present evidence of a climatically unstable yd based on pollen climate transfer function and redfit spectral analysis of pollen data from haligu in the hengduan mountains of southwestern china a low latitude temperate biodiversity hotspot our data show that the climate fluctuated significantly during the yd with three warm peaks recognized at 11 5 11 7 and 12 3 cal kyr bp in particular we estimate a gradual rise in mean annual temperature by 3 7 c between the end of the yd and the early holocene 11 2 10 9 cal kyr bp in response to this the subalpine conifer forest dominated by picea and abies may have migrated about 610 m upwards we also detected that the climate fluctuations during the yd holocene transition show significant centennial periodicities of 76 yr 84 yr 91 yr 95 yr and 638 yr the former four cycles are possibly related to the gleissberg around 80 yr solar cycle which was also detected in the hongyuan peat the pc 6 core of the east china sea and the gisp2 ice core these new findings enhance our understanding of millennial scale abrupt climate events which may be of great significance in predicting the impact of future climate change on mountainous ecosystems in southwestern china the warming at the end of the yd may provide a potential analog for global warming in the near future during which the upward migration of vegetation zones will likely lead to the loss of habitat for threatened species we therefore should take action to restore habitats and provide protected areas for species at risk

quantification of biotic responses to rapid climatic changes around the younger dryas a synthesis to assess the presence or absence of lags in biotic responses to rapid climatic changes we 1 assume that the 18o in biogenically precipitated carbonates record global or hemispheric climatic change at the beginning and at the end of the younger dryas without any lag at our two study sites of gerzensee and leysin switzerland 2 derive a time scale by correlating the 18o record from these two sites with the 18o record of the grip ice core 3 measure 18o records in ostracods and molluscs to check the record in the bulk samples and to detect possible hydrological changes 4 analyse at gerzensee and leysin as well as at two additional sites that lack carbonates and hence a 18o record pollen plant macrofossils chironomids beetles and other insects and cladocera 5 estimate our sampling resolution using the grip time scale for the isotope stratigraphies and the biostratigraphies and 6 summarise the major patterns of compositional change in the biostratigraphies by principal component analysis or correspondence analysis we conclude that at the major climatic shifts at the beginning and end of the younger dryas hardly any biotic lags occur within the sampling resolution of 8 30 years and that upland vegetation responded as fast as aquatic invertebrates we suggest that the minor climatic changes associated with the gerzensee and preboreal oscillations were weakly recorded in the biostratigraphies at the lowland site but were more distinct at higher altitudes individualistic responses of plant and animal species to climatic change may reflect processes in individuals e g productivity and phenology in populations e g population dynamics in spatial distributions e g migrations and in ecosystems e g trophic state we suggest that biotic responses may be telescoped together into relatively short periods 50 to 150 years perhaps disrupting functional interactions among species and thus destabilising ecosystems

paleoclimate reconstruction along the pole equator pole transect of the americas pep 1 examples are presented of inter hemispheric comparison of instrumental climate and paleoclimate proxy records from the americas for di erent temporal scales despite a certain symmetry of seasonal precipitation patterns along the pep 1 transect decadal variability of winter precipitation shows di erent characteristics in terms of amplitude and frequency in both the last 100 and last 1000 years such di erences in variability are also seen in a comparison of time series of di erent el nin8 o southern oscillation proxy records from north and south america however these di erences do not appear to a ect the spatial correlation with paci c sea surface temperature patterns local and regional di erences in response to climate change are even more pronounced for records with lower temporal resolution and inter hemispheric synchroneity may or may not be indicative of the same forcing this aspect is illustrated in an inter hemispheric comparison of the last 1000 years of glacier variability and of the full and lateglacial lake level history 1999 elsevier science ltd all rights reserved

vegetation dynamics at raraku lake catchment easter island during the past 34 000 years abstract easter island is a paradigmatic example of human impact on ecosystems the role of climate changes in recent vegetation shifts has commonly been rejected without proper assessment a palynological study of a long sediment core from raraku lake documents the vegetation dynamics for the last 34 ka and investigates their driving forces particularly the effects of climate variability on vegetation changes significant relationships between pollen assemblage changes and sedimentary and geochemical proxies demonstrate the rapid response of vegetation to lake crater basin hydrology and climatic changes the lake surroundings were occupied by an open mixed palm grove during the last glacial period poaceae and sophora increased at the expense of palms and triumfetta and coprosma practically disappeared in response to slightly wetter and or colder climate during the last glacial maximum palms and triumfetta thrived in a warmer and or drier climate during the deglaciation minor vegetation changes a slight increase in sophora and a drop in asteraceae and poaceae occurred between 13 2 and 11 8 cal ka bp and can be related to rapid changes in the younger dryas chronozone the increase in herbaceous taxa indicates a gradual shallowing of the lake and development of a mire during the holocene caused by sediment infilling and warmer and drier climate relatively rapid vegetation changes in the holocene were caused by climate and by plant succession on the expanding mire the rates of vegetation change observed in the mire were similar to those at the initial stages of human impact identified in a previous study these results reveal significant vegetation changes prior to human presence due to the interplay of climate variations temperature and moisture changes in lake basin form by infilling and intrinsic dynamics of plant succession hence the potential contribution of these factors in vegetation shifts during the period of human presence should not be neglected

was the younger dryas triggered by a flood draining of a huge lake into the northern atlantic may have triggered a cold period 12 900 years ago the route taken by the flood waters remains unknown

late quaternary deglacial history of the merida andes venezuela radiocarbon dated sediment cores from seven lakes and two bogs spanning the cordillera de merida in the venezuelan andes were used to identify and date the regional history of late pleistocene and holocene glacial activity coring sites were selected at different elevations across a pronounced rain shadow from southeast wet to northwest dry sediment lithostratigraphy and magnetic susceptibility in conjunction with ams radiocarbon dates on macrofossils and char coal were used to constrain deglaciation the local expression of the last glacial maximum occurred between 22 750 and 19 960 cal yr bp on the wetter southeastern side of the cordillera de merida glaciers had significantly retreated by 15 700 cal yr bp followed by several minor glacial advances and retreats between 14 850 and 13 830 cal yr bp at least one major glacial readvance occurred between 13 830 and 10 000 cal yr bp in the wetter southeastern sector of the region the drier northwest side of the cordillera de merida records initial glacial retreat by 14 240 cal yr bp multiple sites on both sides of the merida andes record a further phase of extensive deglaciation approximately 10 000 cal yr bp however the north northwest facing mucubajo catchment remained partially glaciated until ca 6000 cal yr bp deglacial ages from the venezuelan andes are consistently younger than those reported from the southern hemisphere andes suggesting an inter hemispheric deglacial lag in the northern tropics of the order of two thousand years copyright 2005 john wiley sons ltd

little ice age clearly recorded in northern greenland ice cores four ice cores drilled in the little investigated area of northern and northeastern greenland were evaluated for their isotopic 18o and chemical content from these rather uniform records a stable isotope temperature time series covering the last 500 years has been deduced which reveals distinct climate cooling during the 17th and the first half of the 19th century timing of the preindustrial temperature deviations agrees well with other northern hemisphere temperature reconstructions however their extent 1 c significantly exceeds both continental records as well as previous southern and central greenland ice core time series a 20 30 increase in the sea salt aerosol load during these periods supports accompanying circulation changes over the north atlantic comparison with records of potential natural climate driving forces points to an important role of the long term solar influence but to only episodically relevant cooling during years directly following major volcano eruptions

disentangling abrupt deglacial hydrological changes in northern south america insolation versus oceanic forcing paleoenvironmental studies and climate models demonstrate that fl uvial runoff and moisture availability in the caribbean hinterland react very sensitively to climatic variations late pleistocene and holocene climate records document pronounced dry and wet periods over tropical south america mainly caused by shifts of the intertropical convergence zone itcz however forcing mechanisms for changes in the itcz position remain controversial here we present high resolution foraminiferal ba ca and 18oseawater records from a core located within the orinoco river outfl ow documenting abrupt hydrological changes in the orinoco catchment area during the deglacial and holocene our data obtained from the surface dwelling foraminifera globigerinoides ruber pink show an abrupt increase in ba ca ratios in the early holocene starting 600 yr after the end of the younger dryas yd cold interval at ca 10 8 ka and suggesting a massive reorganization of moisture sources in northern south america in contrast the salinity dependent 18oseawater from the same samples shows a gradual decrease starting at the end of the yd the offset of our ba ca peak excludes meltwater release in conjunction with the northern andean glacier retreat well before the end of the yd as a forcing mechanism we suggest that the ba ca record documents an abrupt increase in ba rich waters of a northern andean source caused by the insolation driven shift of the itcz and or enhanced monsoon activity

proglacial lake sediment records reveal holocene climate changes in the venezuelan andes lake sediment records from the cordillera de merida in the northern venezuelan andes document the history of local glacial variability and climate changes during the holocene w12 ka to the present the valleys that contain these lakes have similar bedrock compositions and hypsometries but have different headwall elevations and aspects which makes them useful for investigating the magnitude of past glaciations there was widespread glacial retreat in the venezuelan andes during the early holocene after which most watersheds remained ice free and thus far only valleys with headwalls higher than w4400 m asl contain evidence of glaciation during the last w10 ka there was a pronounced shift in sediment composition for the montos headwall w4750 m asl and los anteojos headwall w4400 m asl records during the middle holocene from w8 0 to 7 7 ka when conditions appear to have become ice free and drier there is tentative evidence that the glacier in the mucubaji valley headwall w4609 m asl advanced from w8 1 to 6 6 ka and then retreated during the latter stages of the middle holocene clastic sediment accumulation in other nearby lake basins was either low or decreased throughout most of the middle holocene as watersheds stabilized under warmer and or drier conditions in the montos record there was another major shift in sediment composition that occurred from w6 5 to 5 7 ka similar to other regional records that suggest conditions were drier during this period overall the late holocene was a period of warmer and wetter conditions with ice extent at a minimum in the northern tropical andes there were also punctuated decadal to multi centennial periods of higher clastic sediment accumulation during the last w4 ka likely in response to periods of cooling and or local precipitation changes in watersheds with headwalls above 4600 m asl there is evidence of glacial advances during the little ice age w0 6e0 1 ka the pattern of glacial variability is generally similar in both the northern and southern tropics during the little ice age suggesting that ice margins in both regions were responding to colder and wetter conditions during the latest holocene the observed pattern of holocene climate variability in the venezuelan andes cannot be explained by insolation forcing alone and tropical ocean influences were likely associated with the observed glacial and lake level changes

testing provenance diagrams lessons from the well constrained cariaco basin abstract the fine grained sediments of the cariaco basin venezuela of the last 130 ky whose deposition history is well characterized were analyzed geochemically in order to test the validity of sediment bulk geochemistry as an indicator of detrital provenance several binary and ternary diagrams as well as the chemical index of alteration cia were tested for their capacity to discriminate the poorly contrasted detrital sources to the cariaco basin and to describe the temporal evolution of the contributions of these different sources most of the diagrams tested did not allow a good discrimination of sources or when sources were well discriminated did not allow an interpretation of the temporal variations consistent with the known history a relatively good discrimination of sources and a consistent interpretation of temporal variations were however obtained using hf vs th and la yb vs gd yb binary diagrams as well as ti zr th ti zr la and lu hf th ternary diagrams compared to the previous studies of the detrital content of the cariaco basin sediments the geochemical approach permitted the recognition of a sediment contribution eroded from the unare platform and gulf of cariaco during rapid sea level oscillations and the contribution of saharan eolian particles during the younger dryas preboreal and mis6 5 transition the choice of plotted elements was determined after considering carrier minerals so that different elements may be informative in different sedimentary contexts overall mineral sorting during transport appears as a major limit to quantitative estimation of the different contributions in particular mineral sorting leads to the selective enrichment of elements associated with clays al rb th and lree in sediments deposited in the basin unless the geochemical effect of mineral sorting can be measured it appears that quantitative provenance analysis should be performed on fractions of similar grain size instead of bulk sediment

palynological signal of the younger dryas in the tropical venezuelan andes the occurrence or not of the younger dryas cold reversal in the tropical andes remains a controversial topic this study reports a clear signal for this event in the venezuelan andes employing high resolution palynological analysis of a well dated sediment core from laguna de los anteojos situated around 3900 m elevation within grass paramo vegetation the lake is surrounded by some polylepis forests which are close to their upper distribution limit the section of the core discussed here is 150 cm long and dated between about 14 68 and 9 35 cal kyr bp using a polynomial age depth model based on six ams radiocarbon dates between 12 86 and 11 65 cal kyr bp an abrupt shift occurred in the pollen assemblage manifested by a decline of podocarpus polylepis and huperzia combined with an increase in poaceae and asteraceae the aquatic pteridophyte isoetes also decreased and disappeard and the algae remains show their minimum values pollen assemblages from the younger dryas interval show maximum dissimilarity values compared with today s pollen assemblage and are more similar to modern analogs from superparamo vegetation growing at elevations 400 500 m higher a lowering of vegetation zones of this magnitude corresponds to a temperature decline of between 2 5 and 3 8 c during this colder interval lake levels may have been lower suggesting a decrease in available moisture the vegetation shift documented in anteojos record between 12 86 and 11 65 cal kyr bp is comparable to the el abra stadial in the colombian andes but it differs in magnitude the anteojos shift is better dated and coincides with the younger dryas chron as recorded in the cariaco basin sea surface temperature reconstructions and records of continental runoff as well as in the oxygen isotope measurements from the greenland ice cores when compared to other proxies of quasi immediate response to climate the time lag for the response of vegetation to climate is found to be negligible at a centennial scale the polylepis pollen curve is especially noteworthy as it reproduces the overall pollen trends and matches well with paleoclimatic reconstructions based on other proxies hence polylepis might be used as a reliable paleoclimatic indicator in lake sediments close to its uppermost distribution boundary

vegetation and landscape dynamics under natural and anthropogenic forcing on the azores islands a 700 year pollen record from the sao miguel island abstract the azores archipelago has provided significant clues to the ecological biogeographic and evolutionary knowledge of oceanic islands palaeoecological records are comparatively scarce but they can provide relevant information on these subjects we report the palynological reconstruction of the vegetation and landscape dynamics of the sao miguel island before and after human settlement using the sediments of lake azul the landscape was dominated by dense laurisilvas of juniperus brevifolia and morella faya from ca 1280 ce to the official european establishment 1449 ce after this date the original forests were replaced by a complex of erica azorica myrsine africana forests shrublands and grassy meadows which remained until ca 1800 ce extractive forestry cereal cultivation rye maize wheat and animal husbandry progressed until another extensive deforestation ca 1774 ce followed by the large scale introduction 1845 ce of the exotic forest species cryptomeria japonica and pinus pinaster which shaped the present day landscape fire was a significant driver in these vegetation changes the lake levels experienced a progressive rise during the time interval studied reaching a maximum by ca 1778 1852 ce followed by a hydrological decline likely due to a combination of climatic and anthropogenic drivers our pollen record suggests that sao miguel were already settled by humans by ca 1287 ce approximately one century and a half prior to the official historically documented occupation of the archipelago the results of this study are compared with the few palynological records available from other azores islands pico and flores

forest savanna morichal dynamics in relation to fire and human occupation in the southern gran sabana se venezuela during the last millennia the southern gran sabana se venezuela holds a particular type of neotropical savanna characterized by the local occurrence of morichales mauritia palm swamps in a climate apparently more suitable for rain forests we present a paleoecological analysis of the last millennia of lake chonita 4 39 n 61 0 w 884 m elevation based on biological and physico chemical proxies savannas dominated the region during the last millennia but a significant vegetation replacement occurred in recent times the site was covered by a treeless savanna with nearby rainforests from 3640 to 2180 cal yr bp water levels were higher than today until about 2800 cal yr bp forests retreated since about 2180 cal yr bp onwards likely influenced by a higher fire incidence that facilitated a dramatic expansion of morichales the simultaneous appearance of charcoal particles and mauritia pollen around 2000 cal yr bp supports the potential pyrophilous nature of this palm and the importance of fire for its recent expansion the whole picture suggests human settlements similar to today in which fire is an essential element since around 2000 yr ago therefore present day southern gran sabana landscapes seem to have been the result of the synergy between biogeographical climatic and anthropogenic factors mostly fire

evidence of the younger dryas in central america abstract three sites provide records that address whether the younger dryas was manifest in central america marine records from barbados and the cariaco basin are discussed as well as possible climatic forcing mechanisms sequences from montane la chonta bog costa rica and lowland lake quexil guatemala record a temperature decline of 1 5 2 5 c dated between 11 070 10 400 14c years bp and ca 12 000 10 300 14c years bp respectively the record from lake la yeguada panama does not include cooling although significant reorganization of local communities occurred during the younger dryas chronozone colder tropical ssts throughout deglaciation contributed to sub regional climatic variation more intense monsoonal activity and cooler air temperatures however the temperature reversals are not associated with the coldest tropical ssts which occurred during meltwater pulses the temperature reversals appear to be a response to extra tropical conditions presumably in the north atlantic which suggests that younger dryas cooling extended as far south as costa rica in panama monsoonal conditions may have blocked cooler temperatures precipitation continued to increase throughout deglaciation due to strong forcing by seasonal solar insolation in central america while desiccation events occurred to the north and south

evidence for a younger dryas glacial advance in the andes of northwestern venezuela abstract deposits of push moraine outwash and glaciolacustrine sediments recovered from two areas in the northwestern venezuelan andes document the latest pleistocene advance of merida ice underlying peats provide maximum ages on till and outwash evidently emplaced during the younger dryas yd climatic event one example recovered from the humboldt massif where the farthest extent of yd ice buried peat in the surface of late glacial till provides a within glacier advance age of 12 4 ka cal bp the peat lies on moraine deposited during a stillstand event when the humboldt glacier retreated to the area of lago verde at 4000 m a s l approximately 0 5 km upvalley toward lago suero yd till buries peat deposited in glaciolacustrine sediments of presumed late glacial age farther north in the mucunuque mucubaji catchment of the eastern cordillera a push moraine of possible yd age buries older till of late glacial age 0 2 km upvalley outwash of yd age buries glaciolacustrine peat and organic rich alluvial sediment dated to 13 7 and 13 3 ka cal bp the latest merida advance documented here is approximately synchronous with the yd cold event of europe and the north atlantic region ca 11 6 12 7 ka cal bp the yd event in both areas of the northwestern venezuelan andes nearly reestablished earlier late glacial ice positions and termination appears to have been abrupt the valleys in both areas were evacuated of yd ice without emplacement of recessional moraines as during the main deglaciation at the humboldt site equilibrium line altitudes ela s for the late glacial were about 50 m lower than during the inferred yd in the mucunuque mucubaji catchment ela s for the late glacial 3900 m a s l are difficult to establish given the absence of lateral moraines

early human occupation and land use changes near the boundary of the orinoco and the amazon basins se venezuela palynological evidence from el pauji record this paper shows a holocene paleoecological reconstruction based on a peat bog sequence el pauji 4 28 n 61 35 w 865 m elevation located in the transition zone between the gran sabana se venezuela savannas and the amazon rainforests paleoecological trends are based on the analysis of pollen and pteridophyte spores algal and animal remains fungal spores and charcoal particles the whole record embraces the last ca 8000 cal yr bp and was subdivided into five pollen zones representing the following vegetation succession savanna rainforest mosaic 8250 7715 yr bp dense rainforests 7715 5040 yr bp savanna rainforest mosaic 5040 2690 yr bp secondary dry forests 2690 1440 yr bp and peat bog in an open savanna landscape 1440 yr bp present these vegetation changes have been attributed to the action of climate and or land use changes as well as the corresponding synergies between them fire has been determinant in the landscape evolution based on the reconstructed fire and vegetation shifts a changing land use pattern could have been recognized between the early and the mid holocene ca 8 3 5 0 kyr bp land use practices seem to have been more linked to shifting agriculture in a rainforest landscape as is usual in amazon cultures with medium fire incidence affecting only local forest spots or surrounding savannas more extensive forest burning was recorded between ca 5 0 and 2 7 kyr bp followed by land abandonment and the dominance of drier climates between 2 7 and 1 4 yr bp the modern indigenous culture which prefers open environments and makes extensive use of fire thus preventing forest re expansion seem to have established during the last 1500 yr therefore a significant cultural replacement has been proposed for the region leading to the present day situation changing human activities have been instrumental for ecological evolution in this savanna rainforest transitional region as well as for the shaping of modern landscapes

the deglaciation of the americas during the last glacial termination this paper reviews current understanding of deglaciation in north central and south america from the last glacial maximum to the beginning of the holocene together with paleoclimatic and paleoceanographic data we compare and contrast the pace of deglaciation and the response of glaciers to major climate events during the global last glacial maximum glgm 26 5 19 ka average temperatures decreased 4 to 8 c in the americas but precipitation varied strongly throughout this large region many glaciers in north and central america achieved their maximum extent during the glgm whereas others advanced even farther during the subsequent heinrich stadial 1 hs 1 glaciers in the andes also expanded during the glgm but that advance was not the largest except on tierra del fuego hs 1 17 5 14 6 ka was a time of general glacier thickening and advance throughout most of north and central america and in the tropical andes however glaciers in the temperate and subpolar andes thinned and retreated during this period during the bolling allerod interstadial b a 14 6 12 9 ka glaciers retreated throughout north and central america and in some cases completely disappeared many glaciers advanced during the antarctic cold reversal acr 14 6 12 9 ka in the tropical andes and patagonia there were small advances of glaciers in north america central america and in northern

rapid oceanic and atmospheric changes during the younger dryas cold period the younger dryas event which began approximately 12 900 years ago was a period of rapid cooling in the northern hemisphere driven by large scale reorganizations of patterns of atmospheric and oceanic circulation 1 3 environmental changes during this period have been documented by both proxy based reconstructions 3 and model simulations 4 but there is currently no consensus on the exact mechanisms of onset stabilization or termination of the younger dryas 5 8 here we present high resolution records from two sediment cores obtained from lake krakenes in western norway and the nordic seas multiple proxies from lake krakenes are indicative of rapid alternations between glacial growth and melting during the later younger dryas meanwhile reconstructed sea surface temperature and salinity from the nordic seas show an alternation between sea ice cover and the influx of warm salty north atlantic waters we suggest that the influx of warm water enabled the westerly wind systems to drift northward closer to their present day positions the winds thus brought relatively warm maritime air to northern europe resulting in rising temperatures and the melting of glaciers subsequent input of this fresh meltwater into the ocean spurred the formation of sea ice which forced the westerly winds back to the south cooling northern europe we conclude that rapid alternations between these two states immediately preceded the termination of the younger dryas and the permanent transition to an interglacial state the impact of freshwater pulses released from northern hemisphere ice sheets on the north atlantic meridional overturning circulation has been suggested as a likely mechanism for the younger dryas cooling 7 11 in addition a responsive sea ice cover has been proposed as a mechanism that could rapidly alter the heat exchange between the ocean and atmosphere during the last glacial period 12 13 the roles of the ocean and the atmosphere in triggering the climate shifts around the younger dryas are much discussed 9 these hypotheses are difficult to resolve partly because the temporal resolution of most palaeoclimate archives is too coarse to enable the anatomy of the younger dryas stadial to be adequately described 14 to address these questions we here present a lake sediment record at sub annual resolution together with new data on the sea surface conditions of the norwegian atlantic current nwac throughouttheyoungerdryasstadial fig 1

potential glacial evidence for the younger dryas event in the cordillera of north and south america there is debate as to whether or not the younger dryas yd climatic event affected areas outside of the north atlantic region the potential of alpine glacial deposits to record the yd is of interest because alpine glaciers would be likely to leave a morainal record even during minor short lived advances caused by a period of deteriorating climate in the cordilleran ranges of north and south america there are several candidate moraines many bracketed between radiocarbon ages of ca 12 and 10 ka which may be yd in age the best examples are in the canadian and american rockies the ecuadorian peruvian and bolivian andes and around the patagonian ice cap in most cases temporal brackets are not narrow enough and some radiocarbon ages may not be accurate enough to allow correlation at the present time there are two cases of lateglacial readvance whose ages are interpreted to match or fall within the yd interval an advance of the crowfoot glacier in canada is interpreted to have occurred within the period ca 11 3 and 10 14c ka bp and an advance of the reschreiter glacier in ecuador is interpreted to have culminated in the period ca 11 to 10 6 14c ka bp the latter interpretation depends on an assumption that lake level variations were caused by glacier advances and retreats

a younger dryas icecap in the equatorial andes abstract morphologic and stratigraphic evidence shows that a late glacial ice cap existed on part of the eastern cordillera of ecuador lat 0 20 s on ground with a mean elevation of 4200 m where none exists now an outlet glacier from an ca 800 km 2 ice cap terminated at 3850 m altitude in the papallacta valley on the eastern side of the plateau radiocarbon dates show that moraines formed by this advance were ice free by 13 200 14 c yr b p tephras and the age of organic deposits at the plateau edge indicate ice free conditions before 11 800 14 c yr b p this interval was followed by the expansion of an ca 140 km 2 ice cap that discharged glaciers into adjacent valleys where terminal moraines were built at 3950 m altitude ams and conventional radiocarbon dates from macrofossils peat and gyttja above and below till of the readvance indicate that the ice cap formed between ca 11 000 and 10 000 14 c yr b p and was thus coeval with the european younger dryas event the ice cap developed in response to a surface temperature cooling of at least 3 c in the tropical andes a finding that is consistent with a coupled equatorial high latitude north atlantic climate system operating at the late glacial holocene transition these results are further evidence that younger dryas cooling may have been a global event

the last glacial holocene transition in southern chile warming at the last glacial termination in the north atlantic region was interrupted by a period of renewed glacial activity during the younger dryas chronozone ydc the underlying mechanism of this cooling remains elusive but hypotheses turn on whether it was a global or a north atlantic phenomenon chronological sedimentological and palaeoecological records from sediments of small lakes in oceanic southern chile demonstrate that there was no ydc cooling in southern chile it is therefore likely that there was little or no cooling in southern pacific surface waters and hence that ydc cooling in the north atlantic was a regional rather than global phenomenon

rapid locking of tectonic magnetic fabrics in weakly deformed mudrocks abstract the anisotropy of magnetic susceptibility ams has provided important insights into early deformation conditions in compressive settings through characterization of tectonic fabrics in mudrocks that appear otherwise undeformed the validity of these insights relies on the assumption that tectonic fabrics are rapidly locked shortly after sediment deposition however the time lag between deposition and the tectonic overprint has yet to be quantified to verify this assumption here we present ams data from late holocene sediments recovered in two cores from lake issyk kul in the kyrgyz tien shan fold and thrust belt these sediments in which magnetic fabrics reflect the preferred orientation of phyllosillicates have typical tectonic magnetic fabrics with varying degrees of tectonic overprint likely controlled by small scale active faults the mean orientation of the susceptibility maxima parallels neighbouring active thrust faults and is perpendicular to the geodetically derived local shortening direction the age model for one of the studied cores which is based on 7 calibrated accelerator mass spectrometer radiocarbon dates indicates that sediments as young as 25 yr record this clear tectonic fabric which is locked within sediments older than 1180 cal yr bp our demonstration of rapid locking of tectonic fabrics in weakly deformed mudrocks shortly after deposition provides the required validation of their reliability for tectonic studies and opens new opportunities for providing quantitative insights into the relationship between magnetic ellipsoids shortening rates and stress directions in compressive settings

evidence from the northwestern venezuelan andes for extraterrestrial impact the black mat enigma abstract a carbon rich black layer encrusted on a sandy pebbly bed of outwash in the northern venezuelan andes previously considered the result of an alpine grass fire is now recognized as a black mat candidate correlative with clovis age sites in north america falling within the range of black mat dated sites 12 9 ka cal bp as such the bed at site mum7b which dates to 14 c years bp raw dates and appears to be contemporaneous with the younger dryas yd cooling event marks a possibly much more extensive occurrence than previously identified no fossils megafauna or tool assemblages were observed at this newly identified candidate site 3800 a m s l as in the case of the north american sites here evidence is presented for an extraterrestrial impact event at 12 9 ka the impact related andean bed located 20 cm above 13 7 13 3 ka cal bp alluvial and glaciolacustrine deposits falls within the sediment characteristics and age range of black mat dated sites 12 9 ka cal bp in north america site sediment characteristics include carbon glassy spherules magnetic microspherules carbon mat welded onto coarse granular material occasional presence of platinum group metals rh and ru planar deformation features pdfs in fine silt size fragmental grains of quartz as well as orthoclase and monazite with an abundance of rare earth elements rees if the candidate site is black mat correlative with the black mat sites of north america such an extensive occurrence may support the hypothesized airburst impact over the laurentide glacier which led to a reversal of allerod warming and the onset of yd cooling and readvance of glaciers while this finding does not confirm such it merits further investigation which includes the reconnaissance for additional sites in south america furthermore if confirmed such an extensive occurrence may corroborate an impact origin

vegetation changes in the neotropical gran sabana venezuela around the younger dryas chron the occurrence of the younger dryas cold reversal in northern south america midlands and lowlands remains controversial we present a palaeoecological analysis of a late glacial lacustrine section from a midland lake lake chonita 4 6501 8n 61 0157 8w 884 m elevation located in the venezuelan gran sabana based on physical and biological proxies the sediments were mostly barren from 15 3 to 12 7 k cal a bp probably due to poor preservation a ligneous community with no clear modern analogues was dominant from 12 7 to 11 7 k cal a bp younger dryas chronozone at present similar shrublands are situated around 200 m elevation above the lake suggesting a cooling driven downward shift in vegetation during that period the interval from 11 7 to 10 6 k cal a bp is marked by a dramatic replacement of the shrubland by savannas and a conspicuous increase in fire incidence the intensification of local and regional fires at this interval could have played a role in the vegetation shift a change to wetter and probably warmer conditions is deduced after 11 7 k cal a bp coinciding with the early holocene warming these results support the hypothesis of a mixed origin climate and fire of the gran sabana savannas and highlight the climatic instability of the neotropics during the late glacial copyright 2011 john wiley sons ltd

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

tropical ice core records evidence for asynchronous glaciation on milankovitch timescales over the last 28 years ice core records have been systematically recovered from ten high elevation ice fields nine of which are located in the low latitudes each core has provided new information about the regional climate and environmental change and together their records challenge existing paradigms about the earth s climate system when viewed collectively these ice core histories provide compelling evidence that the growth glaciation and decay deglaciation of large ice fields in the lower latitudes are often asynchronous both between the hemispheres and with high latitude glaciation that occurs on milankovitch timescales although stable isotopic records suggest that global scale cooling occurred during the last glacial stage lgs we contend that precipitation is the primary driver of glaciation in the low latitudes this is consistent with the time transgressive nature of precession driven changes in insolation and hence precipitation such that glaciers advance retreat in the tropics north of the equator while glaciers retreat advance in the tropics south of the equator thus the coeval inter hemispheric retreat of glaciers in the 20th century and the beginning of the 21st century is atypical copyright 2005 john wiley sons ltd

hydrology and climatology at laguna la gaiba lowland bolivia complex responses to climatic forcings over the last 25 000 years diatom geochemical and isotopic data provide a record of environmental change in laguna la gaiba lowland bolivia 17 45 s 57 35 w over the last ca 25 000 years high resolution diatom analysis around the last glacial interglacial transition provides new insights into this period of change the full and late glacial lake was generally quite shallow but with evidence of periodic flooding at about 13 100 cal a bp just before the start of the younger dryas chronozone the diatoms indicate shallower water conditions but there is a marked change at about 12 200 cal a bp indicating the onset of a period of high variability with rising water levels punctuated by periodic drying from ca 11 800 to 10 000 cal a bp stable deeper water conditions persisted there is evidence for drying in the early to middle holocene but not as pronounced as that reported from elsewhere in the southern hemisphere tropics of south america this was followed by the onset of wetter conditions in the late holocene consistent with insolation forcing conditions very similar to present were established about 2100 cal a bp a complex response to both insolation forcing and millennial scale events originating in the north atlantic is noted

a diatom based holocene palaeoenvironmental record from a mid arctic lake on boothia peninsula nunavut canada a 485 cm sediment core from a lake unoffcially called jr01 boothia peninsula nunavut canada yielded a high resolution diatom record documenting environmental change in the mid arctic five radi

climatic change during the younger dryas chron in northern south america a test of the evidence abstract new ams and palynological data are presented from the colombian andes to assess vegetational and climatic change during the lateglacial holocene transition with special emphasis on the younger dryas yd chronozone the new evidence is compared and discussed with other colombian cores and with data from other countries in the region the lateglacial climatic reversal in colombia known as the el abra stadial has an estimated lower boundary of 11 200 conventional and 10 900 interpolated 14 c yr bp respectively although the el abra stadial is assumed to be the equivalent of the european yd the present data suggest that the el abra stadial is a biostratigraphical signal representing both the yd chronozone and the earliest holocene 11 000 9000 14 c yr bp on the basis of new ams dates and a re evaluation of the pollen zones we divided the colombian el abra biozone into two phases from ca 11 000 to ca 10 500 14 c yr bp there is a sharp increase of subparamo and paramo pollen reflecting a relatively cool phase during the yd chronozone zone y1 after ca 10 500 14 c yr bp a slight increase of arboreal pollen and the presence of cactaceae zone z1 point toward a relatively milder but drier phase extending to ca 9000 14 c yr bp in the earliest holocene our conclusions add detail to the concept of astronomical forcing of contrasting rainfall changes in northern south america between 12 400 and 8800 14 c yr bp we propose an environmental drought during the el abra biozone with subsequent erosion of the sediments deposited during that period as a major factor explaining the poor presence of yd evidence in northern south america we conclude that at hydrologically sensitive sites without a clear lithological change around 11 000 9000 14 c yr bp a hiatus during the yd chronozone can only be detected if other cores are available with bracketed time control we state that the start of the temperature decline in central america and northern south america is related to the global yd cooling event however much work is still required to understand the duration and amplitude of the climatic reversal during the yd chronozone in this part of the globe

stress analysis in the intraplate area of gazli uzbekistan from different sets of earthquake focal mechanisms occurrence of intraplate earthquakes have been related either to reactivation of preexisting zones of weakness in a uniformly oriented regional stress field or to local stress changes more recently buildup of pore pressure anomalies has been proposed to explain both the low recurrence rate and some misorientation of rupture planes in the prevailing regional stress field in this study we analyze local stress field orientation in the area of the three ms 7 0 gazli western uzbekistan earthquakes by using different sets of earthquakes focal mechanisms we show that stress tensor inversions from both seven 4 2 ms 7 0 forty five 0 1 ml 3 2 and twenty five 1 0 ml 3 2 earthquake focal mechanisms give constant maximum horizontal stress direction in agreement with the one deduced from the india asia collision the best model for the local stress tensor exhibits a significant 25 discrepancy from vertical for one of the principal stresses we note moreover that the common assumption that one principal stress axis is oriented vertically is not strictly supported by stress states derived from earthquake focal mechanisms available in the world stress database by evaluating each hypothesis of stress inversion methods from earthquake focal mechanisms it is shown that especially in intraplate settings the first motion focal solution should be the most appropriate one for a formal inversion and that the magnitude of the considered event may not be the only criterion to assess the quality of a stress inversion this is particularly timely because more and more focal mechanisms are moment tensor solutions instead of first motion solutions concerning the gazli sequence the slip conditions pore pressure and frictional coefficient are evaluated for each of the three mainshocks in terms of an independently determined local stress field because variations in orientations between local and regional stress field are not detected and because the three mainshock fault planes seem to be relatively well oriented for a frictional activation in the local stress field it is concluded that this seismic sequence is localized on preexisting weakness zones and is not due to local stress orientation anomalies

climate induced variations in productivity and planktonic ecosystem structure from the younger dryas to holocene in the cariaco basin venezuela a high resolution molecular organic geochemical study of sediments in the anoxic cariaco basin indicates significant changes in primary productivity and planktonic community structure associated with the transition from the younger dryas to the holocene variations in climate conditions over the past 12 14c kyr have induced large scale changes in upwelling intensity which directly affected levels of primary productivity as reflected in accumulation rates of bulk productivity proxies concentrations and accumulation rates of sterol and alkenone biomarkers have been used to identify how productivity changes affected the structure of the planktonic ecosystem a shift in the dominant primary producer from diatoms younger dryas to coccolithophores holocene is identified if productivity and ecosystem variations like those identified in the tropical upwelling zone of the cariaco basin region occur throughout the tropical oceans they have the potential to affect global climate through perturbations in the biogeochemical cycle of carbon

vegetation changes and human settlement of easter island during the last millennia a multiproxy study of the lake raraku sediments earlier palynological studies of lake sediments from easter island suggest that the island underwent a recent and abrupt replacement of palm dominated forests by grasslands interpreted as a deforestation by indigenous people however the available evidence is inconclusive due to the existence of extended hiatuses and ambiguous chronological frameworks in most of the sedimentary sequences studied this has given rise to an ongoing debate about the timing and causes of the assumed ecological degradation and cultural breakdown our multiproxy study of a core recovered from lake raraku highlights the vegetation dynamics and environmental shifts in the catchment and its surroundings during the late holocene the sequence contains shorter hiatuses than in previously recovered cores and provides a more continuous history of environmental changes the results show a long gradual and stepped landscape shift from palm dominated forests to grasslands this change started c 450 bc and lasted about two thousand years the presence of verbena litoralis a common weed which is associated with human activities in the pollen record the significant correlation between shifts in charcoal influx and the dominant pollen types suggest human disturbance of the vegetation therefore human settlement on the island occurred c 450 bc some 1500 years earlier than is assumed climate variability also exerted a major influence on environmental changes two sedimentary gaps in the record are interpreted as periods of droughts that could have prevented peat growth and favoured its erosion during the medieval climate anomaly and the little ice age respectively at c ad 1200 the water table rose and the former raraku mire turned into a shallow lake suggesting higher precipitation evaporation rates coeval with a cooler and wetter pan pacific ad 1300 event pollen and diatom records show large vegetation changes due to human activities c ad 1200 other recent vegetation changes also due to human activities entail the introduction of taxa e g psidium guajava eucalyptus sp and the disappearance of indigenous plants such as sophora toromiro during the two last centuries although the evidence is not conclusive the american origin of v litoralis re opens the debate about the possible role of amerindians in the human colonisation of easter island

spatial and temporal variability of holocene temperature in the north atlantic region the early holocene climate of the north atlantic region was influenced by two boundary conditions that were fundamentally different from the present the presence of the decaying laurentide ice sheet lis and higher than present summer solar insolation in order to assess spatial and temporal patterns of holocene climate evolution across this region we collated quantitative paleotemperature records at sub millennial resolution and synthesized their temporal variability using principal components analysis pca the analysis reveals considerable spatial variability most notably in the time transgressive expression of the holocene thermal maximum htm most of the region but especially areas peripheral to the labrador sea and hence closest to the locus of lis disintegration experienced maximum holocene temperatures that lagged peak summer insolation by 1000 3000 years many sites from the northeastern north atlantic sector including the nordic seas and scandinavia either warmed in phase with maximum summer insolation 11 000 9000 years ago or were less strongly lagged than the baffin bay labrador sea region these spatially complex patterns of holocene climate development which are defined by the pca resulted from the interplay between final decay of the lis and solar insolation forcing

response to comment on absence of cooling in new zealand and the adjacent ocean during the younger dryas chronozone applegate et al present the results of a moraine degradation model and suggest that the age of the waiho loop may be 1000 years older than the age we presented thus raising the possibility that the moraine is a younger dryas landform we show that this assessment is misleading on a number of grounds

rapid ice margin fluctuations during the younger dryas in the tropical andes abstract radiocarbon dated lacustrine sequences in peru show that the chronology of glaciation during the late glacial in the tropical andes was significantly out of phase with the record of climate change in the north atlantic region fluvial incision of glacial lake deposits in the cordillera blanca central peru has exposed a glacial outwash gravel radiocarbon dates from peat stratigraphically bounding the gravel imply that a glacier advance culminated between 11 280 and 10 990 14c yr b p rapid ice recession followed similarly in southern peru ice readvanced between 11 500 and 10 900 14c yr b p as shown by a basal radiocarbon date of 10 870 14c yr b p from a lake within 1 km of the quelccaya ice cap by 10 900 14c yr b p the ice front had retreated to nearly within its modern limits thus glaciers in central and southern peru advanced and retreated in near lockstep with one another the younger dryas in the peruvian andes was apparently marked by retreating ice fronts in spite of the cool conditions that are inferred from the 18o record of sajama ice this retreat was apparently driven by reduced precipitation which is consistent with interpretations of other paleoclimatic indicators from the region and which may have been a nonlinear response to steadily decreasing summer insolation

a high mountain pollen altitude calibration set for palaeoclimatic use in the tropical andes the numerical relationship between modem pollen assemblages and altitude in high mountain environments from the northern andes is analysed in order to found inference models that allow estimating palaeoaltitudes and palaeotemperatures from past pollen records the calibration set dm consists of a 50 sample altitudinal transect between 2300 and 4600 m altitude the overall and individual pollen responses to altitude were tested by correspondence analysis ca generalized linear regression hof and weighted averaging wa transfer functions were derived by weighted averaging partial least squares wa pls regression overall altitude is the main controlling factor for the composition of pollen assemblages as shown by the high correlation between altitude and the first ca component r 0 88 individually around 35 of the 82 pollen taxa show a significant response to altitude through monotonic or unimodal functions the best transfer function obtained has a good statistical performance as shown b

holocene thermal maximum in the western arctic 0 180 w the spatio temporal pattern of peak holocene warmth holocene thermal maximum htm is traced over 140 sites across the western hemisphere of the arctic 0 180 w north of 60 n paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer than present conditions at 120 of these sites at the 16 terrestrial sites where quantitative estimates have been obtained local htm temperatures primarily summer estimates were on average 1 6 0 8 c higher than present approximate average of the 20th century but the warming was time transgressive across the western arctic as the precession driven summer insolation anomaly peaked 12 10 ka thousands of calendar years ago warming was concentrated in northwest north america while cool conditions lingered in the northeast alaska and northwest canada experienced the htm between ca 11 and 9 ka about 4000 yr prior to the htm in northeast canada the delayed warming in quebec and labrador was linked to the residual laurentide ice sheet which chilled the region through its impact on surface energy balance and ocean circulation the lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change the spatial asymmetry of warming during the htm resembles the pattern of warming observed in the arctic over the last several decades although the two warmings are described at different temporal scales and the htm was additionally affected by the residual laurentide ice the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing unlike the htm however future warming will not be counterbalanced by the cooling effect of a residual north american ice sheet

comment on absence of cooling in new zealand and the adjacent ocean during the younger dryas chronozone barrows et al reports 5 october 2007 p 86 presented cosmogenic exposure dates and data from an ocean sediment core that challenge evidence for glacier advance in new zealand during the younger dryas event we use modeling of geomorphic processes to argue that their cosmogenic exposure dates are inconclusive

the el abra stadial a younger dryas equivalent in colombia abstract a survey of radiocarbon dated palynological stratigraphical and glacio morphological evidence from colombia is presented showing a cold climatic fluctuation most likely between 10 900 and 10 100 bp immediately preceding the initiation of the holocene this fluctuation called the el abra stadial is characterized by lower temperatures and less precipitation and occurs after the late glacial guantiva interstadial average annual temperature during the guantiva interstadial may have been 2 c lower than today and during the el abra stadial from 4 to 6 c lower than today the upper forest line during the el abra stadial was some 400 to 500 m lower than during the guantiva interstadial and some 600 to 800 m lower than today the guantiva interstadial el abra stadial early holocene sequence is radiocarbon dated by some 45 dates and evidence is shown from 14 areas mostly located at high elevations 2000 4000 m in the eastern and central cordillera and in the sierra nevada de santa marta but some from the tropical low lands although more precise dating of the boundaries is desirable we interpret the guantiva el abra climatic fluctuations as equivalents in time and climatic phenomena to the allerod younger dryas sequence in europe

environmental processes in rano aroi easter island peat geochemistry forced by climate variability during the last 70kyr abstract we analyze the geochemistry of rano aroi mire record easter island using bulk peat composition c n s and stable isotopes 13c 15n 34s and major minor and trace elemental compositions obtained by icp aes al ti zr sc v y fe mn th ba ca mg and sr peat geochemistry and the pollen record are used to reconstruct the environmental changes during the last 70 kyr bp principal component analysis on icp aes data revealed that three main components account for the chemical signatures of the peat the first component characterized by lithogenic elements combined signal of v al sc y cr cd ti zr and cu evidences long term changes in the basal fluxes of mineral material into the mire this component in combination with stable isotopes and pollen data suggests a link between soil erosion and vegetation cover changes in the rano aroi watershed the second component is identified by the signal of fe mn th ba zr and ti and is indicative of strong runoff events during enhanced precipitation periods the third component tied mainly to ca sr and mg reflects a strong peat oxidation event that occurred during an arid period with more frequent droughts sometime between 39 and 31 kyr bp correlation coefficients and a multiple regression model pcr analysis between peat organic chemistry and the principal components of icp aes analysis were calculated isotope chemistry of the peat organic matter further contributes to define rano aroi environmental history 13c data corroborates a vegetation shift documented by the palynological record from c4 to c3 between 55 and 45 cal kyr bp the 15n record identifies periods of changes in mire productivity and denitrification processes while the 34s peat signature indicates a marine origin of s and significant diagenetic cycling the geochemical and environmental evolution of rano aroi mire is coherent with the regional climatic variability and suggests that climate was the main forcing in mire evolution during the last 70 kyr bp the coupling of geochemical and biological proxies improves our ability to decipher depositional processes in tropical and subtropical peatlands and to use these sequences for paleoenvironmental and paleoclimate reconstructions

abrupt younger dryas cooling in the northern tropics recorded in lake sediments from the venezuelan andes a radiocarbon dated sediment record from laguna de los anteojos a cirque lake in the merida andes of venezuela indicates that warmer and wetter atmospheric conditions occurred in the northern tropics at the onset of the bolling 14 600 cal yr bp and abruptly colder and drier conditions around the time of the younger dryas yd geochemical and clastic sediment analyses from los anteojos show that glaciers advanced at 12 850 cal yr bp reached their yd maximum extent at 12 650 cal yr bp and then retreated until complete deglaciation of the watershed at 11 750 cal yr bp the onset of warmer conditions that ended the coldest phase of the yd occurred several hundred years earlier at los anteojos than in the high latitudes of the northern hemisphere during the peak yd glacial advance glacier equilibrium line altitudes in the region were 360 to 480 m lower and temperature was 2 2 to 2 9 c colder than modern independent palynological evidence from the los anteojos sediment core indicates that the northern andes were more arid and at least 2 3 c colder during the yd the direction and timing of glacial fluctuations in venezuela are consistent with observations of marine sediment records from the cariaco basin that suggest abrupt cooling occurred at 12 850 cal yr bp followed by a shift to higher temperature after 12 300 cal yr bp the timing and pattern of climatic changes in northern south america are also consistent with paleoclimate records from the southern tropical andes that suggest a southward shift in the position of the intertropical convergence zone occurred at the start of the cooling event followed by a return to wetter conditions in northern south america during the late stages of the yd the early warming of the tropical atmosphere and invigoration of the hydrologic cycle likely contributed to the shift to increased temperature in the higher latitudes of the northern hemisphere at the end of the late glacial stage

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

absence of cooling in new zealand and the adjacent ocean during the younger dryas chronozone as the climate warmed at the end of the last glacial period a rapid reversal in temperature the younger dryas yd event briefly returned much of the north atlantic region to near full glacial conditions the event was associated with climate reversals in many other areas of the northern hemisphere and also with warming over and near antarctica however the expression of the yd in the mid to low latitudes of the southern hemisphere and the southwest pacific region in particular is much more controversial here we show that the waiho loop advance of the franz josef glacier in new zealand was not a yd event as previously thought and that the adjacent ocean warmed throughout the yd

glaciation in the andes during the lateglacial and holocene this review updates the chronology of andean glaciation during the lateglacial and the holocene from the numerous articles and reviews published over the past three decades the andes which include some of the world s wettest and driest mountainous regions offer an unparalleled opportunity to elucidate spatial and temporal patterns of glaciation along a continuous 68 degree meridional transect the geographic and altitudinal extent of modern glaciers and the sensitivity of both modern and former glaciers to respond to changes in specific climatic variables reflect broad scale atmospheric circulation and consequent regional moisture patterns glaciers in the tropical andes and in the mid latitude andes are likely to have been far more sensitive to changes in temperature than glaciers in the dry subtropical andes broad scale temporal and spatial patterns of glaciation during the lateglacial are apparent in the southernmost andes the lateglacial chronology appears to have a strong antarctic signature with the best dated moraines correlating closely with the antarctic cold reversal the southernmost andes do not appear to have experienced a significant ice advance coeval with the younger dryas yd climatic reversal at the other end of the andes from w 0t o 9 n a stronger yd connection may exist but critical stratigraphic and geochronologic work is required before a yd ice advance can be fully demonstrated in the central andes of peru well dated moraines record a significant ice readvance at the onset of the yd but ice was retreating during much of the remaining yd interval the spatial temporal pattern of holocene glaciation exhibits tantalizing but incomplete evidence for an early to mid holocene ice advance s in many regions but not in the arid subtropical andes where moraines deposited during or slightly prior to the little ice age lia record the most extensive advance of the holocene in many regions there is strong evidence for neoglacial advances in the interval between 1 0 and 2 5 ka moraines that correlate with the lia of the northern hemisphere are seen in all presently glacierized mountain ranges most of these date to within the past 450 years outboard of these moraines in many regions are moraines of a slightly more extensive advance that occurred several hundred years prior to the onset of the lia priorities for future work include filling in several prominent spatial gaps in the distribution of chronologic studies for the lateglacial these gaps include the arid regions of northern chile and argentina the southern peruvian andes between 11 5 and 13 5 s and the andes of northern peru and

climate reconstruction for the last two millennia in central iberia the role of east atlantic ea north atlantic oscillation nao and their interplay over the iberian peninsula abstract a multi proxy characterization of the uppermost sedimentary infill of an iberian alpine lake cimera 2140 m a s l was performed to establish the climatic and environmental conditions for the iberian central range icr over the last two millennia this multi proxy characterization was used to reconstruct the intense runoff events lake productivity and soil erosion in the lake catchment and interpret these factors in terms of temperature and precipitation variability the roman period rp 200 bce 500 ce beginning was characterized by an alternation between cold and warm periods as indicated by short lived oscillations of intense runoff conditions and soil erosion although warm conditions dominated the end of the period and the early middle age ema 500 900 ce onset in the icr a noticeable decrease in intense runoff events and a progressive decrease in soil erosion during the late ema indicated a shift to colder temperatures in terms of precipitation both the rp and ema climate periods displayed a transition from dry to wet conditions that led to a decrease in lake productivity the medieval climate anomaly mca 900 1300 ce was characterized by warm and dry conditions with frequent intense runoff episodes and increases in lake productivity and soil erosion whereas the little ice age lia 1300 1850 ce showed the opposite characteristics the industrial era 1850 2012 ce presented an increase in lake productivity that likely demonstrates the influence of global warming the spatio temporal integration of the cimera record with other iberian reconstructions has been used to identify the main climate drivers over this region during the rp and ema n s and e w humidity gradients were dominant whereas during the mca and lia these gradients were not evident these differences could be ascribed to interactions between the north atlantic oscillation nao and east atlantic ea phases during the rp the general warm conditions and the e w humidity gradient indicate a dominant interplay between a negative nao phase and a positive ea phase nao ea whereas the opposite conditions during the ema indicate a nao ea interaction the dominant warm and arid conditions during the mca and the cold and wet conditions during the lia indicate the interplay of the nao ea and nao ea respectively furthermore the higher solar irradiance during the rp and mca may support the predominance of the ea phase whereas the opposite scenario during the ema and lia may support the predominance of the ea phase which would favour the occurrence of frequent and persistent blocking events in the atlantic region during these periods

a review of lateglacial pollen records from ecuador and peru with reference to the younger dryas event abstract published biostratigraphic records from ecuador spanning the lateglacial period of approximately 13 000 to 10 000 bp lack radiocarbon dates delineating the younger dryas and records with extrapolated dates do not exhibit evidence of a climatic reversal for peru fossil pollen data from four sites suggest one and possibly two climatic oscillations in the lateglacial but only one site laguna junin has dates that delimit a short term climatic change and this occurs between 12 000 and 11 000 bp pollen records from laguna baja and rio blanco pond give the strongest indication of a climatic reversal but bracketing dates are not available other pollen records from peru have tenuous dating or data and do not support a lateglacial climatic reversal at the time of the younger dryas

the mysterious onset of the younger dryas abstract the 1300 year long younger dryas cold reversal 12 900 11 600 cal bp is seen most clearly in greenland ice cores contemporaneous changes of climate and environment in more southerly regions were variably expressed the precise timing of the abrupt younger dryas onset is still uncertain it was linked to a sudden increase in 14c manifest as a cliff where 14c dates drop from 11 000 to 10 600 radiocarbon years bp within a century of real time followed by a long plateau changes in ocean and or atmospheric circulation or solar radiation and even a comet impact have been proposed as triggers but the abruptness and severity of yd onset still elude explanation

synchroneity of tropical and high latitude atlantic temperatures over the last glacial termination a high resolution western tropical atlantic sea surface temperature sst record from the cariaco basin on the northern venezuelan shelf based on mg ca values in surface dwelling planktonic foraminifera reveals that changes in sst over the last glacial termination are synchronous within 30 to 90 years with the greenland ice sheet project 2 air temperature proxy record and atmospheric methane record the most prominent deglacial event in the cariaco record occurred during the younger dryas time interval when ssts dropped by 3 to 4 c a rapid southward shift in the atmospheric intertropical convergence zone could account for the synchroneity of tropical temperature atmospheric methane and high latitude changes during the younger dryas