pollen based quantitative reconstructions of holocene climate variability in nw romania abstract a modern analogue technique is applied to two high resolution pollen sequences from nw romania to provide the first quantitative evidence for winter summer and annual temperatures and for precipitation across the holocene in this region the pollen based climate reconstructions allow the identification of four main intervals i an early less stable period between 11 700 and 11 200 cal yr bp ii generally stable conditions between 11 200 and 8300 cal yr bp with winter and annual temperatures and precipitation higher than at present and summer temperatures about the same iii lower winter and annual temperatures and higher summer temperatures and precipitation between 8000 and 2400 cal yr bp iv warmer winter and annual temperatures and lower precipitation for the last 2400 years whereas summer temperatures became cooler at steregoiu and remained stable at preluca tiganului the pollen based climate reconstructions at the two sites show similar patterns in annual and winter temperatures and precipitation changes during the holocene but the trends appear to be less consistent for summer temperatures our pollen based reconstructions revealed several short term climatic oscillations during the holocene the strongest of which occur between 10 300 10 100 8300 8000 6800 6400 5100 4900 4000 3600 and 3200 3000 cal yr bp

chironomid inferred holocene temperature changes in the south carpathians romania we present a holocene summer air temperature reconstruction based on fossil chironomids from lake brazi 1740 m a s l a shallow mountain lake in the south carpathians summer air temperature rec

holocene climate variability as reflected by mid european lake level fluctuations and its probable impact on prehistoric human settlements abstract a data set of 180 radiocarbon tree ring and archaeological dates obtained from sediment sequences of 26 lakes in the jura mountains the northern french pre alps and the swiss plateau was used to construct a holocene mid european lake level record the dates do not indicate a random distribution over the holocene but form clusters suggesting an alternation of lower and higher climatically driven lake level phases they provide evidence of a rather unstable holocene climate punctuated by 15 phases of higher lake level 11 250 11 050 10 300 10 000 9550 9150 8300 8050 7550 7250 6350 5900 5650 5200 4850 4800 4150 3950 3500 3100 2750 2350 1800 1700 1300 1100 750 650 cal bp and after 1394 ad a comparison of this mid european lake level record with the gisp2 polar circulation index pci record the north atlantic ice rafting debris ird events and the 14 c record suggests teleconnections in a complex cryosphere ocean atmosphere system correlations between the gisp2 pci the mid european lake level the north atlantic ird and the residual 14 c records suggest that changes in the solar activity played a major role in holocene climate oscillations over the north atlantic area

a chironomid based reconstruction of late glacial summer temperatures in the southern carpathians romania late glacial and early holocene summer temperatures were reconstructed based on fossil chironomid assemblages at lake brazi retezat mountains with a joint norwegian swiss transfer function providing an important addition to the late glacial quantitative climate reconstructions from europe the pattern of the late glacial temperature changes in lake brazi show both similarities and some differences from the ngrip 18o record and other european chironomid based reconstructions our reconstruction indicates that at lake brazi 1740 m a s l summer air temperature increased by 2 8 c at the oldest dryas bolling transition gs 2 gi 1 and reached 8 1 8 7 c during the late glacial interstade the onset of the younger dryas gs 1 was characterized by a weak 1 c decrease in chironomid inferred temperatures similarly at the gs 1 holocene transition no major changes in summer temperature were recorded in the early holocene summer temperature increased in two steps and reached 12 0 13 3 c during the preboreal two short term cold events were detected during the early holocene between 11 480 11 390 and 10 350 10 190 cal yr bp the first cooling coincides with the preboreal oscillation and shows a weak 0 7 c temperature decrease while the second is characterized by 1 c cooling both cold events coincide with cooling events in the greenland ice core records and other european temperature reconstructions

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

comparison of linear and nonlinear models for cohesive sediment detachment rill erosion hole erosion test and streambank erosion studies abstractcohesive sediment detachment is typically modeled for channels levees spillways earthen dams and internal erosion by using a linear excess shear stress approach however mechanistic nonlinear detachment models such as the wilson model have recently been proposed in the literature questions exist as to the appropriateness of nonlinear relationships between applied shear stress and the erosion rate therefore the objective of this research was to test the appropriateness of linear and nonlinear detachment models for cohesive sediment detachment using three data sets 1 rill erodibility studies across a limited range of applied shear stress 0 9 21 4 pa 2 hole erosion tests hets across a wide range of applied shear stress 12 6 62 0 pa and 3 streambank erodibility as quantified by jet erosion tests jets for the linear excess shear stress equation and the nonlinear wilson model across a small range of shear stress 1 4 pa the wilson model was also incorporated into the bank st

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

a 13 600 year diatom oxygen isotope record from the south carpathians romania reflection of winter conditions and possible links with north atlantic circulation changes this study provides a continuous lateglacial and holocene record of diatom silica oxygen isotope changes 18odiat in a subalpine lake sediment sequence obtained from the retezat mts taul dintre brazi 1740 m a s l this through flow shallow high altitude lake with a surface area of only 0 4 ha has short water residence time and is predominantly fed by snowmelt and rainwater its 18odiat record principally reflects the oxygen isotope composition of the winter and spring precipitation as diatom blooms occur mainly in the spring and early summer hence changes in 18odiat are interpreted as seasonal scale changes in the amount of winter precipitation low oxygen isotope values 27 28 5 occurred during the lateglacial until 12 300 cal bp followed by a sharp increase thereafter in the holocene 18odiat values ranged from 29 to 31 until 3200 cal bp followed by generally lower values during the late holocene 27 30 short term decreases in the isotopic values were found between 10 140 9570 9000 8500 7800 7300 6300 5800 5500 5000 and at 8015 4400 4000 cal bp after 3200 cal bp a decreasing trend was visible with the lowest values between 3100 2500 and after 2100 cal bp the general trend in the record suggests that contribution of winter precipitation was generally lower between 11 680 and 3200 cal bp followed by increased contribution during the last millennia the late holocene decrease in 18odiat shows good agreement with the speleothem 18o lake level and testate amoebae records from the carpathian mountains that also display gradual 18o decrease and lake level mire water table level rise after 3200 cal bp strong positive correlation with north atlantic circulation and solar activity proxies such as the austrian and hungarian speleothem records furthermore suggested that short term increases in the isotopic ratios in the early and mid holocene are likely connectable to high solar activity phases and high frequency of positive north atlantic oscillation indexes that may have resulted in decreased winter precipitation in this region

holocene and late pleistocene climate in the sub mediterranean continental environment a speleothem record from poleva cave southern carpathians romania the pp10 stalagmite from poleva cave provides a late pleistocene and holocene isotopic record characteristic for the sw of romania a sub mediterranean climatic region the speleothem was dated by eight tims and one alpha u series dates which showed that it was precipitated between 75 ka and 2 ka with at least two hiatuses the basal sector of the stalagmite showed a slow growing regime of 0 26 cm ka while the upper one grew relatively fast with about 5 cm ka the temporal resolution for the isotopic sampling is thus 2 ka sample for the lower sector and 150 years sample for the upper one the relationship between 18 o and temperature was found positive the isotopic record of the lower sector shows two marked cold intervals during 67 and 58 ka and 40 35 ka respectively which correlate well with the villars and soreq records the upper sector record is so far the most detailed holocene isotopic record in romania and the only one available for the regions located at the exterior of the carpathians range the signal shows a gradual warming after the gs1 event punctuated by several cold events at 8 7 2 and 4 2 ka and also by warm oscillations centered at about 5 2 and 3 3 ka the results seem to indicate that if the north atlantic firstorder signals may extend well to the south eastern europe their amplitude and general trend may be diminished by the interferences with the mediterranean circulation

detrital events and hydroclimate variability in the romanian carpathians during the mid to late holocene the romanian carpathians are located at the confluence of three major atmospheric pressure fields the north atlantic the mediterranean and the siberian despite its importance for understanding past human impact and climate change high resolution palaeoenvironmental reconstructions of holocene hydroclimate variability and in particular records of extreme precipitation events in the area are rare here we present a 7500 year long high resolution record of past climatic change and human impact recorded in a peatbog from the southern carpathians integrating palynological geochemical and sedimentological proxies natural climate fluctuations appear to be dominant until 4500 years before present yr bp followed by increasing importance of human impact sedimentological and geochemical analyses document regular minerogenic deposition within the bog linked to periods of high precipitation such minerogenic depositional events began 4000 yr bp with increased depositional rates during the medieval warm period mwp the little ice age lia and during periods of societal upheaval e g the roman conquest of dacia the timing of minerogenic events appears to indicate a teleconnection between major shifts in north atlantic oscillation nao and hydroclimate variability in southeastern europe with increased minerogenic deposition correlating to low nao index values by linking the minerogenic deposition to precipitation variability we state that this link persists throughout the mid to late holocene

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

orbital ice sheet and possible solar controls on holocene moisture trends in the north atlantic drainage basin holocene patterns of hydroclimatic change in the north atlantic region are poorly understood despite the importance to climate of the region9s freshwater flux to the ocean here we reconstruct the history of moisture balance in the north atlantic drainage by compiling lake level records from 95 lakes in europe and north america we show that recent millennia were significantly wetter than much of the holocene in both regions only 15 of lakes studied in the north atlantic basin were low in the past millennium in contrast 50 of lakes were low at 10 cal kyr b p low moisture levels in the early holocene likely resulted from high summer and fall insolation and the influence of the continental ice sheets water levels rose in many areas as these influences diminished a principal components analysis of the lake level data however also indicates that holocene atmospheric circulation changes possibly driven by solar activity generated additional moisture variation including low water levels at 5 2 cal kyr b p in eastern north america and scandinavia

tilt prior to explosions and the effect of topography on ultra long period seismic records at fuego volcano guatemala 1 ground tilt is measured from broadband seismic records prior to frequent explosions at fuego volcano guatemala we are able to resolve tilt beginning 20 30 minutes prior to explosions followed by a rapid reversal in deformation coincident with explosion onsets the tilt amplitude and polarity recorded on the horizontal channels vary from station to station such that the steep and unusual topography of the upper cone of fuego appears to affect the ultra long period signals we account for the effect of topography and attempt to constrain the tilt source depth and geometry through finite difference modeling the results indicate a shallow spherical pressure source and that topography must be considered when attempting to model tilt sources at volcanoes with steep topography the tilt signals are interpreted as pressurization of the shallow conduit beneath a crystallized plug followed by elastic deflation concurrent with explosive pressure release

reconciling diverse diatom based lake responses to climate change in four mountain lakes in the south carpathian mountains during the last 17 kyrs abstract climatic changes were studied using siliceous algae diatoms and chrysophyta stomatocysts analyses in four mountain lakes in the retezat mountains in the south carpathian mountains with the aim to search for synchrony in aquatic ecosystem responses according to the basal radiocarbon dating of the lake sediment cores these lakes were formed around 17 000 15 000 cal yr bp high resolution diatom analyses were carried out together with loss on ignition loi and biogenic silica bisi measurements on the lakes comparison of the proxy results suggests that despite the different slope aspect water depth and basin parameters diatom assemblage changes show clear synchrony the most remarkable changes in the aquatic ecosystems were observed around 6500 cal yr bp on the northern slope and around 6100 cal yr bp on the southern slope during the holocene evidences for sharp concomitant shifts were found between 9200 9000 and 3200 3000 cal yr bp in the siliceous algal communities on both slopes the late glacial holocene boundary was not pronounced in the shallow lakes but was significant in a deep lake the signs of a dry and bipartite younger dryas gs 1 were evident but floristic changes differed in the lakes principal component analyses explained very similar variances along the first and second axes for three lakes while the fourth lake lake bucura proved less sensitive to climate change due to the dominance of moss inhabiting diatom assemblages lake level changes have only been reconstructed on the basis of diatom life forms in one of the deep lakes lake gales high lake levels were inferred between 9300 9000 and 3000 1700 cal yr bp in addition to loi biogenic silica content bisi was measured on the sediments of the northern slope and pointed to different seasonal biological productivities increasing loi was often coupled with low bisi values

using linear discriminant analysis lda of bulk lake sediment geochemical data to reconstruct lateglacial climate changes in the south carpathian mountains abstract a multi proxy approach is widely used for reconstructing climatic change in alpine lake sediments this study applied bulk sediment geochemistry to reconstruct lateglacial and early holocene climatic change in a glacial lake lake brazi 1740 m a s l in the retezat mts south carpathians romania the lowermost 1 m part of a 4 9 m long sediment core covering the period between 9950 and 15 750 cal bp was used for high resolution bulk analysis of major elements al 2 o 3 sio 2 tio 2 cao mgo k 2 o na 2 o fe 2 o 3 mno so 3 linear discriminant analysis lda was used to compare a priori classified main chemical groups subsamples from the core were priory ordered to warm and cold groups respectively according to their age and evidence of cold and warm events in the record as suggested by proxy correlation with the lateglacial event stratigraphy of north greenland ice core project ngrip the discriminant function was calculated using concentrations of al 2 o 3 tio 2 cao mgo k 2 o na 2 o fe 2 o 3 and mno after log ratio transformation loss on ignition silicon and sulphur concentrations were not used for the discriminant analysis but regarded as comparison proxies for checking up the validity of outputs sediments deposited during cold and warm events were separated significantly by the lda function 85 1 of the originally grouped cases were correctly classified under the lda analysis the calculated discriminant scores indicated four cold and four warm events sediments ordered into the warm group contained larger amounts of organic matter and sulphur while sediment samples of the cold group were characterized by the overall predominance of major oxides bound into inorganic silicates the discriminant scores showed strong correlation with the ngrip 18 o data r 0 8135 and with the pollen percentage sum of trees and shrubs r 0 9460 loss on ignition showed a somehow weaker but still conclusive linear relationship with the discriminant scores r 0 7505 discriminant analyses of bulk sediment major oxide chemical data may be a useful tool to identify the impact of climatic events upon the nature and composition of materials delivered to a lake basin

diatom inferred lateglacial and holocene climatic variability in the south carpathian mountains romania climatic and environmental changes were studied using siliceous algae diatoms and chrysophycean cysts analyses in a mountain lake in the south carpathian mountains for the lateglacial and holocene lake brazi 1740 a s l is a small shallow lake in the retezat mountains according to the basal radiocarbon dating of the core the lake was formed around 15 750 cal bp and sediment accumulation has been continuous ever since high resolution diatom analyses were carried out together with loss on ignition and biogenic silica measurements total epilimnetic phosphorus tp concentrations and ph were reconstructed quantitatively ten statistically significant zones were distinguished six of which appeared in the lateglacial and early holocene suggesting the instability of diatom assemblages after deglaciation the uppermost zone covered six thousand years which suggests that diatom assemblages remained quite stable during the second half of the holocene high lake level occurred between 9500 and 9000 cal bp after 6300 cal bp the lake level gradually increased again until 5800 cal bp a further increase was detected at 3000 cal bp while after 1500 cal bp water level decreased although there is limited knowledge regarding the environmental constraints of several diatom taxa present in the sequence e g aulacoseira laevissima which makes the precise environmental interpretation of the diatom record more difficult the available data provide an important new insight into the environmental history of lake brazi and the carpathian mountains the most remarkable changes in the aquatic ecosystem were observed at 6000 cal bp in the holocene and 12 800 cal bp at the onset of the younger dryas marked changes were also detected at 10 500 10 300 9800 9500 9000 8900 8800 8300 6300 5800 and 3100 3000 cal bp

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

climate related eutrophication of a small boreal lake in northwestern ontario a palaeolimnological perspective a palaeolimnological investigation of changes in water quantity and quality in lake 239 experimental lakes area in northwestern ontario indicates marked changes in limnological conditions during the holocene water quantity changes are based on the analysis of diatoms and inferences of lake level changes from a nearshore sediment core changes in water quality are based on the analysis of diatom assemblages and associated quantitative inferences of total phosphorus tp from a deep central core as well as changes in diatom accumulation rates the proportion of chrysophyte scales to diatoms and changes to organic matter content lake levels at least 8 m lower than today were inferred from the nearshore core and were concurrent with an increase in nutrient rich diatom assemblages an increase in diatom accumulation and a decrease in the chrysophytes relative to diatoms in the central core fragilaria crotonensis and aulacoseira subarctica were two of the dominant nutrient rich taxa that contributed to an

early holocene climatic oscillations recorded by lake level fluctuations in west central europe and in central italy abstract on the basis of lake level data the period of the preboreal oscillation pbo at 11 300 11 150 cal yr bp as defined by the grip oxygen isotope record appears to correspond to wetter climatic conditions in west central europe and to a marked drying in north central italy additional short lived phases of higher lake level have been identified before the pbo at ca 11 450 11 400 and 11 350 cal yr bp in west central europe and at ca 11 500 cal yr bp in italy such multiple climatic oscillations around the pbo have been observed in various records in northwestern europe on the basis of various proxies a map of changes in moisture over western europe during the pbo indicates a mid latitude zone between 58 and 43 n characterised by wetter conditions while a drier climate developed in southern and northern europe this wet middle zone shows a larger extension and suggests a more meandering weaker atlantic westerly jet during the pbo than during the 8 2 kyr event a comparison of lake level records in west central europe with 1 outbursts from north american and north european proglacial lakes and 2 variations in solar activity as reflected by 14 c and the 10 be records supports the hypothesis that the pbo was a response to successive meltwater pulses at 11 300 11 250 11 200 and 11 170 cal yr bp and to a sudden decrease in solar activity at 11 250 cal yr bp this study points to the necessity of developing integrated multi proxy approaches to construct more robust regional event stratigraphies and of better documenting palaeohydrological changes in the mediterranean area

rapid vegetation response to lateglacial and early holocene climatic fluctuation in the south carpathian mountains romania abstract high resolution pollen conifer stomata and plant macrofossil analyses of two glacial lake sediments 1740 and 1990 m a s l are used to reconstruct lateglacial lg and early holocene eh vegetation and tree line changes in the retezat mountains our results show that during the lg tree line was between 1750 and 1800 m a s l formed by larix decidua pinus mugo and picea abies early lg spread to high altitudes suggests refugia of these tree species in the mountain the younger dryas cooling resulted in regional steppe tundra expansion but tree line position and composition showed little change the abundance of trees and shrubs decreased at 1740 m a s l but species richness increased with the arrival of pinus cembra our data support climate model hindcasts for only modest decrease in accumulated growing season heat at mid high altitudes regionally the pollen records suggest enhanced aridity and seasonality in the eh tree line reached 2000 m a s l higher than today by 11 100 cal yr bp p mugo p cembra p abies established around the upper lake suggesting rapid increase in summer temperatures the eh maximum of l decidua between 11 200 10 600 cal yr bp was connected to high summer insolation high altitude expansion of abies alba between 10 600 10 300 cal yr bp suggested summer mean temperatures 2 8 c higher then today in comparison with other mountain sites in europe lg interstadial tree line was at similar altitude in the s alps and 350 m higher in the pirin mountains lg tree line fluctuation had similar low amplitude in the se alps retezat and pirin mts suggesting relatively weak influence of the north atlantic thermohaline circulation changes on growing season temperatures

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

persistent influence of the north atlantic hydrography on central european winter temperature during the last 9000 years 1 a prominent feature of the n atlantic s holocene climate is a series of centennial and millennial shifts in ocean surface hydrography bond et al 2001 oppo et al 2003 here we compare a climate reconstruction from the central alps for the last 9 000 years derived from the oxygen isotope record in stalagmites with the percentage of hematite stained grains in n atlantic sediments an indicator for the proportion of ice bearing surface water from north of iceland the excellent match between the two curves proves that meteorological conditions in the alpine region responded synchronously to changes in the hydrography of the n atlantic during winter in addition the speleothem record suggests significant multi centennial and millennial variability of n european winters throughout the past 9 000 years that might have been incompletely recorded in tree ring archives

holocene climate variability although the dramatic climate disruptions of the last glacial period have received considerable attention relatively little has been directed toward climate variability in the holocene 11 500 cal yr b p to the present examination of similar to50 globally distributed paleoclimate records reveals as many as six periods of significant rapid climate change during the time periods 9000 8000 6000 5000 4200 3800 3500 2500 1200 1000 and 600 150 cal yr b p most of the climate change events in these globally distributed records are characterized by polar cooling tropical aridity and major atmospheric circulation changes although in the most recent interval 600 150 cal yr b p polar cooling was accompanied by increased moisture in some parts of the tropics several intervals coincide with major disruptions of civilization illustrating the human significance of holocene climate variability c 2004 university of washington all rights reserved

diatom inferred climatic and environmental changes over the last 9000 years from a low arctic nunavut canada tundra lake abstract diatom assemblage changes over the holocene were examined from a 14c dated sediment core retrieved from lake tk 2 a small low arctic lake located ca 200 km north of the forest tundra ecotone in mainland nunavut canada notable changes in the diatom assemblages were recorded throughout the core suggesting that the holocene epoch in this region has been environmentally and climatically dynamic the earliest diatom assemblages ca 9000 cal yr bp were dominated by taxa that are atypical of post glacial assemblages commonly recorded throughout the arctic and may suggest that early holocene conditions at lake tk 2 were relatively warmer a shift to dominance by small benthic opportunistic fragilaria taxa followed after ca 8550 cal yr bp more typical of arctic assemblages during initial lake ontogeny suggesting the onset of cooler more alkaline conditions an abrupt and short lived marked decrease in diatoms between ca 8550 and 8500 cal yr bp with corresponding changes in physical and chemical indicators e g sedimentation rate siliciclastic content organic matter content provides potential evidence for the 8 2 k cooling anomaly an event rarely recognized from other paleolimnological studies in the canadian north following ca 7000 cal yr bp a substantial shift occurred to a more complex and diverse diatom assemblage that now included more acidophilic taxa this compositional change is likely indicative of a natural long term loss of alkalinity in the lake and marks the onset of the holocene thermal maximum consistent with the timing of this warm period for this region the relatively stable diatom assemblage composition during the neoglacial period was punctuated by fluctuations in key species potentially correlative with the so called medieval warm period and little ice age in the most recent sediments the post industrial expansion of the small planktonic cyclotella stelligera complex and a concurrent decline in the heavily silicified aulacoseira lirata complex are similar to shifts that are increasingly being recognized as geographically widespread diatom responses to recent climate warming the lake tk 2 diatom record provides important insights into the holocene environmental history of this understudied region of the canadian arctic furthermore it is one of the few arctic lakes in which the 8 2 k cold event is possibly expressed

early to mid holocene aridity in central chile and the southern westerlies the laguna aculeo record 34 s abstract central chile 32 35 s lies at the northern border of strong westerly influence and thus exhibits a steep precipitation gradient therefore the palaeoclimatic archives in the region are suitable for detecting past moisture changes the study of laguna aculeo 33 50 s 70 54 w presents a multiproxy holocene lake record including sedimentology geochemistry mineralogy pollen diatoms and radiocarbon dating 17 dates results indicate an arid early to mid holocene period about 9500 5700 cal yr b p after 5700 cal yr b p effective moisture increased progressively and around 3200 cal yr b p modern humid conditions were established numerous intercalated clastic layers reflect flood deposition during rainy winters a fluvial unit was deposited shortly before 9000 cal yr b p subsequently flood events were absent until 5700 but have become frequent since 3200 cal yr b p the frequency of flood layers possibly points to weak or no el nino activity during the early and mid holocene with a subsequent increase during the late holocene during the early and mid holocene the westerlies were probably blocked and hence deflected southward by the subtropical high pressure cell higher precipitation during the last 3200 yr seems strongly related to a weakened subtropical high pressure cell with intensified westerlies and possibly increased el nino activity

mid to late holocene climate change an overview the last 6000 years are of particular interest to the understanding of the earth system because the boundary conditions of the climate system did not change dramatically in comparison to larger glacial interglacial changes and because abundant detailed regional palaeoclimatic proxy records cover this period we use selected proxy based reconstructions of different climate variables together with state of the art time series of natural forcings orbital variations solar activity variations large tropical volcanic eruptions land cover and greenhouse gases underpinned by results from general circulation models gcms and earth system models of intermediate complexity emics to establish a comprehensive explanatory framework for climate changes from the mid holocene mh to pre industrial time the redistribution of solar energy due to orbital forcing on a millennial timescale was the cause of a progressive southward shift of the northern hemisphere nh summer position of the intertropical convergence zone itcz this was accompanied by a pronounced weakening of the monsoon systems in africa and asia and increasing dryness and desertification on both continents the associated summertime cooling of the nh combined with changing temperature gradients in the world oceans likely led to an increasing amplitude of the el nin o southern oscillation enso and possibly increasingly negative north atlantic oscillation nao indices up to the beginning of the last millennium on decadal to multi century timescales a worldwide coincidence between solar irradiance minima tropical volcanic eruptions and decadal to multi century scale cooling events was not found however reconstructions show that widespread decadal to multi century scale cooling events accompanied by advances of mountain glaciers occurred in the nh e g in scandinavia and the european alps this occurred namely during the little ice age lia between ad w1350 and 1850 when the lower summer insolation in the nh due to orbital forcing coincided with solar activity minima and several strong tropical volcanic eruptions the role of orbital forcing in the nh cooling the southward itcz shift and the desertifi cation of the sahara are supported by numerous model simulations other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics but there is also evidence that changes in the ocean heat transport took place during the lia at high northern latitudes with possible additional implications for climates of the southern hemisphere sh

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

a global perspective on last glacial maximum to holocene climate change while the abrupt climate events of the last deglaciation are well defined in ice core records from the polar regions of both hemispheres their manifestation elsewhere is less well constrained here we compile 104 high resolution paleoclimate records to characterize the timing and spatial pattern of climate change during the last deglaciation this compilation indicates relatively concurrent timing of the last glacial maximum lgm peak glacial conditions and the altithermal peak interglacial conditions in the northern 22 1 4 3 ka and 8 0 3 2 ka and southern 22 3 3 6 ka and 7 4 3 7 ka hemispheres suggesting the hemispheres were synchronized by greenhouse gases local insolation and or northern hemisphere induced ocean circulation changes the magnitude of the glacialeinterglacial temperature change increases with latitude reflecting the polar amplification of climate change with a likely minimum global mean cooling of ww4 9 c during the lgm relative to the altithermal empirical orthogonal function eof analysis of 71 records spanning 19e11 ka indicates that two modes explain 72 of deglacial climate variability eof1 61 of variance shows a globally near uniform pattern with its principal component pc1 strongly correlated with changes in atmospheric co2 e of2 11 of variance exhibits a bipolar seesaw pattern between the hemispheres with its principal component pc2 resembling changes in atlantic meridional overturning circulation strength eof analysis of 90 records from 15 to 11 ka indicates that northern and southern modes of climate variability characterize the younger dryas bolling allerod interval these modes dominate at the higher latitudes of each hemisphere and exhibit a complex interaction in the tropics the magnitude of the younger dryas climate anomaly cooler drier increases with latitude in the northern hemisphere with an opposite pattern warmer wetter in the southern hemisphere reflecting a general bipolar seesaw climate response global mean temperature decreased by w0 6 c during the younger dryas therefore our analysis supports the paradigm that while the younger dryas was a period of global climate change it was not a major global cooling event but rather a manifestation of the bipolar seesaw driven by a reduction in atlantic meridional overturning circulation strength

an event stratigraphy for the last termination in the north atlantic region based on the greenland ice core record a proposal by the intimate group it is suggested that the grip greenland ice core should constitute the stratotype for the last termination based on the oxygen isotope signal in that core a new event stratigraphy spanning the time interval from ca 22 0 to 11 5 k grip yr bp ca 19 0 10 0 k 14 c yr bp is proposed for the north atlantic region this covers the period from the last glacial maximum through termination 1 of the deep ocean record to the pleistocene holocene boundary and encompasses the last glacial late glacial of the traditional northwest european stratigraphy the isotopic record for this period is divided into two stadial episodes greenland stadials 1 gs 1 and 2 gs 2 and two interstadial events greenland interstadials 1 gi 1 and 2 gi 2 in addition gi 1 and gs 2 are further subdivided into shorter episodes the event stratigraphy is equally applicable to ice core marine and terrestrial records and is considered to be a more appropriate classificatory scheme than the terrestrially based radiocarbon dated chronostratigraphy that has been used hitherto 1998 john wiley sons ltd

holocene fire regime changes near the treeline in the retezat mts southern carpathians romania abstract to investigate holocene vegetation and fire disturbance histories in the treeline ecotone macroscopic charcoal plant macrofossil and pollen records from two lacustrine sediment records were used lake lia is on the southern slope and lake brazi is on the northern slope of the west east oriented retezat mountain range in the romanian carpathians the records were used to reconstruct holocene fire return intervals fris and biomass burning changes biomass burning was highest at both study sites during the drier and warmer early holocene suggesting that climate largely controlled fire occurrence fuel load also influenced the fire regime as shown by the rapid biomass burning changes in relation to timberline shifts overall the number of inferred fire episodes was smaller on the northern than on the southern slope fris were also comparatively longer 1000 4000 years on the northern slope where picea abies dominated woodlands persisted around lake brazi throughout the holocene on the southern slope where pinus mugo was more abundant around lake lia fris were significantly shorter 80 1650 years a period of frequent fire episodes occurred around 1900 1300 cal yr bp on the southern slope when chironomid inferred summer temperatures increased and the pollen record documents increased anthropogenic activity near the treeline however the forest clearance by burning to increase grazing land was subdued in comparison to other european regions