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

a regional scale climate reconstruction of the last 4000 years from lakes in the nebraska sand hills usa abstract high resolution paleohydrological reconstructions were carried out in five shallow lakes in the nebraska sand hills across an east west transect in order to 1 determine whether long term droughts of the past 4000 years were spatially and temporally coherent across the region 2 distinguish local variation in climate or hydrology from regional patterns of change and 3 compare the paleolimnological results with the existing dune inferred drought records diatom inferred lake level was reconstructed for all sites and compared with other regional records alterations between high and low lake levels were frequent during the past 4000 years which suggests that shifts between dry and wet periods were prevalent across the sand hills extended multi decadal to centennial scale droughts were more common prior to 2000 years bp while the last two millennia were hydrologically more variable and climate conditions alternated on shorter timescales despite some discrepancies among the five records the paleohydrological reconstructions refine the holocene drought history of the nebraska sand hills particularly between 2200 and 4000 cal a bp many of the observed drought events are contemporaneous with severe droughts documented at sites in the northern great plains and rocky mountains lending support for the severity and regional significance of these events in western north america

rise in frequency of surface melting at siple dome through the holocene evidence for increasing marine influence on the climate of west antarctica 1 a new melt layer history from siple dome west antarctica indicates notable late holocene summertime warming visual stratigraphic analyses of the 1004 m ice core identified 62 years with melt layers melting events began around 11 7 ka followed by a period of no melting from 8 8 6 6 ka melt layer frequency increased from 6 6 ka to the present with the 1000 year average melt layer frequency reaching a maximum of 2 at 0 8 ka we use our millennial scale archive of melt events as a unique seasonal paleothermometer to elucidate changes in west antarctic holocene summer climate our calibration suggests the change in melt frequency from 0 to 2 may represent a summer temperature increase of 2 c from the middle to late holocene this temperature change cannot be explained entirely by local change in ice elevation or summer insolation and is in contrast to east antarctic climate records which show peak warmth in the early holocene followed by stable or decreasing temperature we interpret the rise in melt frequency as evidence of an increasing marine influence on the ross sea sector of west antarctica although the surface elevation of siple dome has not changed greatly the continued lateral retreat of the west antarctic ice sheet from its last glacial maximum configuration across the outer continental shelf and the delayed drawdown in ice thickness from the adjacent coastal marie byrd land region in conjunction with periods of increased cyclogenesis perhaps related to variations in enso would allow a moderated maritime climate to more easily reach west antarctica

changes in the parkland boreal forest boundary in northwestern ontario over the holocene abstract changes in vegetation were tracked from a well dated sediment core from a boreal lake lake 239 at 200 year resolution over the holocene this presently oligotrophic lake is located 100 km east from the present day parkland forest ecotone in northwestern ontario near shore sediment core transects from lake 239 have previously shown this lake was at least 8 m lower than present in the mid holocene or 58 less lake volume in comparison to today large shifts were expected in the terrestrial vegetation if the low lake levels were related to climate the core from lake 239 shows increases in the relative abundance and concentration of pollen such as cupressaceae and ambrosia indicating a more open boreal forest between 4500 8000 cal yr bp pollen based inferences of average summer and winter temperatures suggest that temperatures were on average up to 1 2 c warmer than today with winter temperatures up to 4 c warmer the pollen inferences also suggest enhanced precipitation likely in the summer but with an overall increase in evaporation and evapotranspiration resulting in reduced effective moisture to assess regional climate changes pollen based reconstructions of temperature and precipitation were developed and synthesized from sediment cores from eight previously published lakes from which pollen sites were available to both the west and east of lake 239 spanning present day prairie lakes to forested lakes up to 300 km east of the prairie boreal ecotone all sites show shifts in pollen assemblages that indicate a warm mid holocene period prairie sites west of the experimental lakes area ela show mid holocene decreases in precipitation relative to today whereas sites near or east of ela show consistent increases in precipitation but with increased temperatures and enhanced evaporation during the mid holocene

dating methods and geomorphic evidence of palaeoenvironmental changes at the eastern margin of the south peruvian coastal desert 14 30 s before and during the little ice age abstract the atacama desert is known to be one of the driest places on earth however in the study area between ica and nasca south peru 75 76 w 14 15 s palaeoclimate proxies show several semi arid phases during the holocene a more humid period during the early holocene favoured an open grass land which allowed the accumulation of desert loess eitel b hecht s machtle b schukraft g kadereit a wagner g kromer b unkel i reindel m 2005 geoarchaeological evidence from desert loess in the nazca palpa region southern peru palaeoenvironmental changes and their impact on pre columbian cultures archaeometry 47 137 158 the paracas and nasca cultures existing in that region between the 9th century bc and 7th century ad had propitious conditions to evolve and to settle close to the rivers fed by the monsoonal precipitation in the cordillera occidental culminating aridity after 600 ad might have caused the collapse of the nasca civilization the onset of semi arid climate towards the end of the late intermediate period 14th century ad allowed a reoccupation of the area here we present 14c and osl data from sediments of river terraces a debris flow and loess snails scutalus sp indicating appreciable geomorphic activity between 1390 and 1714 cal ad this phase is isochronous to the little ice age period on the northern hemisphere and to an increase in the ice accumulation on the quelccaya glacier in the peruvian altiplano thompson l g mosley thompson e bolzan j f koci b r 1985 a 1500 year record of tropical precipitation in ice cores from the quelccaya ice cap peru science 229 971 973

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

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

geochemistry of daihai lake sediments inner mongolia north china implications for provenance sedimentary sorting and catchment weathering abstract to advance the understanding of sediment distribution catchment weathering hydraulic sorting and sediment provenance in a tectonically stable basin the geochemistry of surface sediment samples from daihai lake in north china is presented mud bulk sediments were analyzed for 10 major and 30 trace elements organic carbon and nitrogen and for 87sr 86sr ratios in silicate fraction acid insoluble ai and carbonates acid soluble as enrichment in fe2o3 cao mno mgo k2o and p2o5 and their positive correlation with al2o3 rb sr ratios and bivariate discrimination diagrams of k vs rb and th u vs th suggest low chemical weathering intensity under semiarid conditions in the daihai catchment accumulation of clays in deeper water sites by sedimentary sorting results in an evident distribution of sediment composition with the lake water depth although similar chondrite normalized ree rare earth element patterns of lake sediment samples with lree light rare earth element enrichment negative eu anomalies and an almost flat hree heavy rare earth element pattern may suggest similar sedimentary source rocks original sediment provenance can be recognized on the basis of their distinctive 87sr 86sr ai ratios and immobile th zr ti and nb trace element signature separating from weathering and sedimentary processes as well as from grain size effect

possible linkages of late holocene drought in the north american midcontinent to pacific decadal oscillation and solar activity oxygen isotopic composition d 18 o the d 18 o record of the past 3100 years reveals that droughts of greater severity and duration than during the 20th century occurred repeatedly especially prior to 300 ad drought variability was anomalously low during the 20th century 90 of the variability values during the last 3100 years were greater than the 20th century average d 18 o is strongly correlated with the index of the pacific decadal oscillation pdo during the past 100 years and periodicities of the lateholocene d 18 o record are similar to those of the pdo furthermore time series of d 18 o and atmospheric d 14 ca re generally coherent after 700 ad both the pacific climate and solar irradiance probably played a role in drought occurrence but their effects were non stationary through the late holocene citation tian j d m nelson and f s hu 2006 possible linkages of late holocene drought in the north american midcontinent to pacific decadal oscillation and solar activity geophys res lett 33 l23702 doi 10 1029 2006gl028169

synchronous climatic change inferred from diatom records in four western montana lakes in the u s rocky mountains late holocene environmental and climatic conditions were reconstructed from diatom assemblages in sediment cores from four western montana lakes crevice lake foy lake morrison lake and reservoir lake the lakes show synchroneity in timing of shifts in diatom community structure but the nature of these changes differs among the lakes two of the sites provide highly resolved records of hydrologic balance while the other two stratigraphic sequences primarily record temperature impact on lake thermal structure all four lakes show significant change in five discrete intervals 2200 2100 1700 1600 1350 1200 800 600 and 250 cal yr bp the similarities in the timing of change suggest overlying regional climatic influences on lake dynamics the 800 600 cal yr bp shift is evident in other paleorecords throughout the great plains and western us associated with the transition from the medieval climate anomaly to the little ice age large scale climatic mechanisms that influence these lake environments may result from atmospheric circulation patterns that are driven by interactions between pacific and atlantic sea surface temperatures which are then locally modified by topography

drought reconstructions for the continental united states abstract the development of a 2 lat 3 long grid of summer drought reconstructions for the continental united states estimated from a dense network of annual tree ring chronologies is described the drought metric used is the palmer drought severity index pdsi the number of grid points is 154 and the reconstructions cover the common period 1700 1978 in producing this grid an automated gridpoint regression method called point by point regression was developed and tested in so doing a near optimal global solution was found for its implementation the reconstructions have been thoroughly tested for validity using pdsi data not used in regression modeling in general most of the gridpoint estimates of drought pass the verification tests used in addition the spatial features of drought in the united states have been faithfully recorded in the reconstructions even though the method of reconstruction is not explicitly spatial in its design the drought reconstructions show that the 1930s dust bow

greenland ice core signal characteristics an expanded view of climate change the last millenium of earth history is of particular interest because it documents the environmental complexities of both natural variability and anthropogenic activity we have analyzed the major ions contained in the greenland ice sheet project 2 gisp 2 ice core from the present to 674 a d to yield an environmental reconstruction for this period that includes a description of nitrogen and sulfur cycling volcanic emissions sea salt and terrestrial influences we have adapted and extended mathematical procedures for extracting sporadic e g volcanic events secular trends and periodicities found in the data sets finally by not assuming that periodic components signals were stationary and by utilizing evolutionary spectral analysis we were able to reveal periodic processes in the climate system which change in frequency turn on and turn off with other climate transitions such as that between the little ice age and the medieval warm period

a diatom record of late holocene climate variation in the northern range of yellowstone national park usa the changes in diatom species composition in a sediment core from crevice lake yellowstone national park spanning the past 2550 yr were used to reconstruct long term limnological and ecological conditions that may be related to late holocene climate variability planktic forms dominate the fossil diatom assemblages throughout this record but changes in species dominance indicate varying nutrient levels over time particularly phosphorus the changes in the nutrient concentrations in the lake were probably driven by changes in temperature and wind strength that affected the duration of water column mixing and thus the extent of nutrient recycling from deep waters prior to 2100 cal before present bp stephanodiscus minutulus and synedra tenera dominated suggesting long cool springs with extensive regeneration of phosphorus from the hypolimnion that resulted from isothermal mixing from 2100 to 800 cal bp these species were replaced by cyclotella michiganiana and cyclotella bodanica these species are characteristic of lower nutrient concentrations and are interpreted here to reflect warm summers with long periods of thermal stratification from 800 to 50 cal bp s minutulus dominated the diatom assemblage suggesting a return to lengthy mixing during spring the most dramatic late holocene changes in the fossil diatom assemblages occurred during the transition from the medieval period to the little ice age approximately 800 cal bp

diatom inferred changes in effective moisture during the late holocene from nearshore cores in the southeastern region of the winnipeg river drainage basin canada the winnipeg river drainage basin wrdb within the boreal forest region of northwest ontario is a region that is expected to be negatively affected by climate warming inferences of droughts over the past two millennia from little raleigh lake were based on two nearshore sediment cores the core locations were from depths of 12 and 15 m and were based on sufficient nearshore sediment accumulation and distance from the modern benthic to planktonic diatom boundary where a distinct shift from dominance of benthic taxa changed to dominance of planktonic taxa in surficial sediments at 11 8 m diatom inferred depth was based on a model developed from 60 surficial sediments within the study lake depth inferences indicate that prolonged periods of aridity occurred from ad 950 to 1300 corresponds to medieval climate anomaly and from ad 1625 to 1750 aridity during little ice age we found that the core collected from a depth closer to the benthic to planktonic diatom boundary was more sensitive to changes in lake level than the deeper core where planktonic diatoms dominated the assemblage the inferred low water stands of the past two millennia are well outside of the range of the past 100 years suggesting that recent drought history may not be a good estimate of future extremes

a dendroclimatic reconstruction of annual precipitation on the western canadian prairies since a d 1505 from pinus flexilis james abstract tree ring chronologies from living and sub fossil pinus flexilis james limber pine trees extend back more than 500 yr on the western edge of the great plains in alberta living trees were found to be as old as 526 yr tree ring growth in pinus flexilis is most sensitive to annual precipitation over the annualization period august to july the longevity of the trees coupled with their sensitivity to variations in annual precipitation make them an important resource for constructing annually resolved multicentury records of paleohydrology on the northwestern great plains a 487 yr reconstruction of annual precipitation in southwestern alberta was produced from the tree ring series this is the longest dendroclimatological reconstruction of precipitation available from the northern plains the reconstruction has some similarity to reconstructions from the western great plains of the united states however there are also many divergences in the timing of droughts some of this divergence may reflect the importance of orographic uplift in producing precipitation along the eastern flanks of the rocky mountains our reconstruction shows that the frequency of droughts in alberta during the period of instrumental records about the last 100 yr has not been appreciably different from conditions of the preceding four centuries in addition the most severe drought of this century which led to widespread farm abandonment between 1918 and 1922 was not the most severe drought in the past 487 yr

the little ice age and medieval warm period in the sargasso sea sea surface temperature sst salinity and flux of terrigenous material oscillated on millennial time scales in the pleistocene north atlantic but there are few records of holocene variability because of high rates of sediment accumulation holocene oscillations are well documented in the northern sargasso sea results from a radiocarbondated box core show that sst was approximately 1 degree c cooler than today approximately 400 years ago the little ice age and 1700 years ago and approximately 1 degree c warmer than today 1000 years ago the medieval warm period thus at least some of the warming since the little ice age appears to be part of a natural oscillation 39 refs 4 figs 1 tab

late holocene flood probabilities in the black hills south dakota with emphasis on the medieval climate anomaly abstract a stratigraphic record of 35 large paleofloods and four large historical floods during the last 2000 years for four basins in the black hills of south dakota reveals three long term flooding episodes identified using probability distributions at a d 120 395 900 1290 and 1410 to present during the medieval climate anomaly a d 900 1300 the four basins collectively experienced 13 large floods compared to nine large floods in the previous 800 years including the largest floods of the last 2000 years for two of the four basins this high concentration of extreme floods is likely caused by one or more of the following 1 instability of air masses caused by stronger than normal westerlies 2 larger or more frequent hurricanes in the gulf of mexico and atlantic ocean and or 3 reduced land covering vegetation or increased forest fires caused by persistent regional drought

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

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

bipolar changes in atmospheric circulation during the little ice age annually dated ice cores from siple dome west antarctica and central greenland indicate that meridional atmospheric circulation intensity increased in the polar south pacific and north atlantic at the beginning 1400 a d of the most recent holocene rapid climate change event the little ice age lia as deduced from chemical concentrations at these core sites the lia was characterized by substantial meridional circulation strength variability and this variability persists today despite strong evidence for an end to lia cooling thus increased late 20th century storm variability may be in part a result of the continuation of these climatic fluctuations

productivity cycles of 200 300 years in the antarctic peninsula region understanding linkages among the sun atmosphere oceans sea ice and biota compared to the rest of the world9s oceans high resolution late holocene paleoclimatic data from the southern ocean are still rare we present a multiproxy record from a sediment core retrieved from a deep basin on the western side of the antarctic peninsula that reveals a dramatic perspective on paleoclimatic changes over the past 3700 yr analyses completed include measurement of magnetic susceptibility and granulometry bed thickness particle size percent organic carbon bulk density and microscopic evaluation of diatom and benthic foraminiferal assemblages and abundances downcore variability of these parameters demonstrates the significance of both short term cycles which recur approximately every 200 yr and longer term events 2500 yr cycles that are most likely related to global climatic fluctuations in the upper 600 cm of the core lower values of magnetic susceptibility ms are correlated with lower bulk density the presence of thinly laminated units specific diatom assemblages and generally higher total organic carbon content below 600 cm magnetic susceptibility is uniformly low though variability in other parameters continues the magnetic susceptibility signal is controlled primarily by dilution of ferromagnetic phases with biosiliceous material this signal may be enhanced further by dissolution of magnetite in the magnetic susceptibility lows high total organic carbon the role of variable primary productivity and its relationship to paleoclimate is assessed through the diatom data in particular magnetic susceptibility lows are characterized by higher than normal abundances of chaetoceros resting spores corethron criophilum and or rhizosolenia spp also are found as is a higher ratio of the most common species of fragilariopsis versus species of thalassiosira these assemblages are indicative of periods of high primary productivity driven by the presence of a meltwater stabilized water column the 200 yr cyclicity noted in other paleoclimatic records around the world suggests a global forcing mechanism possibly solar variability in addition to the cyclic changes in productivity overall elevated productivity is noted below 600 cm or prior to ca 2500 yr b p this increased productivity may represent the tail end of a holocene climatic optimum which is widely recognized in other parts of the world but as yet is poorly documented in antarctica

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

a 2700 year high resolution pollen record of climate change from varved sugan lake in the qaidam basin northeastern tibetan plateau abstract a 2700 year high resolution pollen record from annually varved sugan lake in the qaidam basin at 2793 m a s l was obtained to examine vegetation and climatic change on the ne tibetan plateau pollen data shows that sugan basin was constantly covered by open desert steppe vegetation dominated by chenopodiaceae artemisia poaceae and ephedra however large variations in artemisia chenopodiaceae a c ratios suggest regional moisture fluctuations over the last 2700 years including a dry and relatively stable climate prior to 300 ad relatively wet climate from 300 to 1200 ad with variability during 1100 1200 ad and unstable climate since 1200 ad with relatively moister climate during 1250 1400 ad and 1700 1800 ad however other proxies varve thickness chironomid taxa isotopes of oxygen in precipitated carbonate show fresher water when regional moisture was lower inferred from a c ratio this inconsistency suggests the possible difference of in lake lithology environment and regional moisture change fresh water into the lake from ice melting on the surrounding mountains might have contributed to the in lake lithology and environment variation the effective moisture changes in the basin are in opposite phases to snow accumulation records from dunde ice core 5325 m a s l and to the monsoon intensity inferred from dongge cave suggesting that the regional topography might have played an important role in mediating moisture changes at regional scale pollen data from sugan lake shows the shift of moisture at 1200 ad from stable to variable conditions this event is well correlated with other paleoclimate proxies in china and other parts of the world however the mechanisms behind these patterns require further investigation

physical and chemical predictors of diatom dissolution in freshwater and saline lake sediments in north america and west greenland diatom dissolution in surface sediment samples from two regional lake datasets in the northern great plains ngp n 5 64 and west greenland n 5 40 is assessed using a morphological approach categorizing valves during routine diatom analysis two dissolution indices are derived to parameterize diatom dissolution and when compared between two analysts in a blind test show good correspondence and are closely correlated to diatom fragmentation we explore the relationships between hydrochemical and physical lake parameters including meromixis on dissolution within both lake regions using multivariate methods and modeled with logistic regression salinity is the sole significant predictor of dissolution in west greenland but salinity carbonate concentration co3 22 and meromixis are significant predictors in the ngp limnological parameters explain 40 59 of variation in dissolution in both regions for both dissolution indices the dissolution index methodology is applied to a short sediment sequence from devils lake north dakota where diatom inferred salinity inferences can be compared with a historical record of salinity fluctuations over the 20th century absolute errors in paleosalinity estimates are strongly correlated with diatom dissolution with salinity overestimated in 8 out of 11 poorly preserved samples preservation does appear to constrain the reliability of the inferred paleosalinity at this site and may also affect the quality of diatom based paleoenvironmental inferences elsewhere including estimates of biogenic silica where preservation state is often not explicitly considered

centennial eolian cyclicity in the great plains usa a dominant climate pattern of wind transport over the past 4000 years proxy evidence at decadal resolution from late holocene sediments from pickerel lake northeastern south dakota shows distinct centennial cycles 400e700 years in magnetic susceptibility contents of carbonate organic carbon and major elements abundance in ostracodes and d 18 o and d 13 c values in calcite proxies indicate cyclic changes in eolian input productivity and temperature maxima in magnetic susceptibility are accompanied by maxima in aluminum and iron mass accumulation rates mars and in abundances of the ostracode fabaeformiscandona rawsoni this indicates variable windy and dry conditions with westerly wind dominance including during the medieval climate anomaly maxima in carbonates organic carbon phosphorous and high d 13 c values of endogenic calcite indicate moister and less windy periods with increased lake productivity including during the little ice age and alternate with maxima of eolian transport times of the maunder sporer and wolf sunspot minima are characterized by maxima in d 18 o values and aluminum mars and minima in d 13 c values and organic carbon content we interpret these lake conditions during sunspot minima to indicate decreases in lake surface water temperatures of up to 4e5 c associated with decreases in epilimnetic productivity during

lake sediments record large scale shifts in moisture regimes across the northern prairies of north america during the past two millennia six high resolution climatic reconstructions based on diatom analyses from lake sediment cores from the northern prairies of north america show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia the predominant broad scale pattern seen at all sites is a major shift in moisture regimes from wet to dry or vice versa depending on location that occurred after a period of relative stability these large scale shifts at the different sites exhibit spatial coherence at regional scales the three canadian sites record this abrupt shift between anno domini 500 and 800 and subsequently conditions become increasingly variable all three u s sites underwent a pronounced change but the timing of this change is between anno domini 1000 and 1300 thus later than in all of the canadian sites the mechanisms behind these patterns are poorly understood but they are likely related to changes in the shape and location of the jet stream and associated storm tracks if the patterns seen at these sites are representative of the region this observed pattern can have huge implications for future water availability in this region

on solar forcing of holocene climate evidence from scandinavia the focus of this study is the possible correlation between changes in the climate of scandinavian and changes in solar irradiation reliable information about holocene climatic change in sweden and norway is currently available from two main sources the 14c dating of pine wood retrieved from above the present pine tree limit and studies of glacier variations based on proglacial lacustrine sediments and on moraines the reconstructed alpine tree limit reveals that summer temperature in general was warmer during the early holo cene than it was during the late holocene superimposed on this general trend are several fluctuations of a few hundred years duration relatively cold periods with a duration of the order of 300 600 years occurred frequently during the holocene in this paper dates of the major climatic events are compared with an index of solar activity the so called 14c anomalies for most of the last 9000 years a good correspondence is demonstrated between the timing of cold events in scandi

the little ice age as recorded in the stratigraphy of the tropical quelccaya ice cap the analyses of two ice cores from a southern tropical ice cap provide a record of climatic conditions over 1000 years for a region where other proxy records are nearly absent annual variations in visible dust layers oxygen isotopes microparticle concentrations conductivity and identification of the historical a d 1600 huaynaputina ash permit accurate dating and time scale verification the fact that the little ice age about a d 1500 to 1900 stands out as a significant climatic event in the oxygen isotope and electrical conductivity records confirms the worldwide character of this event

determinants of fire activity during the last 3500 yr at a wildland urban interface alberta canada abstract long term records of wildfires and their controlling factors are important sources of information for informing land management practices here dendrochronology and lake sediment analyses are used to develop a 3500 yr fire and vegetation history for a montane forest in jasper national park alberta canada the tree ring record ad 1771 2012 indicates that this region historically experienced a mixed severity fire regime and that effective fire suppression excluded widespread fire events from the study area during the 20th century a sediment core collected from little trefoil lake located near the jasper townsite is analyzed for subfossil pollen and macroscopic charcoal 150 m when comparing the tree ring record to the 3500 yr record of sediment derived fire events only high severity fires are represented in the charcoal record comparisons between the charcoal record and historical climate and pollen data indicate that climate and vegetation composition have been important controls on the fire regime for most of the last 3500 yr although fire frequency is presently within the historical range of variability the fire return interval of the last 150 yr is longer than expected given modern climate and vegetation conditions indicating that humans have become the main control on fire activity around little trefoil lake

coherent late holocene climate driven shifts in the structure of three rocky mountain lakes large scale atmospheric pressure centers such as the aleutian and icelandic low have a demonstrated relationship with physical lake characteristics in contemporary monitoring studies but the responses to these phenomena are rarely observed in lake records we observe coherent changes in the stratification patterns of three deep 30 m lakes inferred from fossil diatom assemblages as a response to shifts in the location and intensity of the aleutian low and compare these changes with similar long term changes observed in the 18o record from the yukon specifically these records indicate that between 3 2 and 1 4 ka the aleutian low shifted westward resulting in an increased frequency of storm tracks across the pacific northwest during winter and spring this change in atmospheric circulation ultimately produced deeper mixing in the upper waters of these three lake systems enhanced stratification between 4 5 and 3 3 ka and from 1 3 ka to present suggests a strengthened aleutian low and more meridion

possible solar forcing of century scale drought frequency in the northern great plains a 2100 yr decadal resolution salinity and aridity proxy record of lacustrine ostracode shell mg ca ratios from a closed basin lake in the northern great plains shows statistically significant periodicities of 400 200 130 and 100 yr these periodicities are similar to the three principal solar oscillation periods 420 218 143 yr as inferred from the atmospheric radiocarbon record suggesting strong solar forcing of century scale drought frequency with a fundamental 400 yr period and its 2nd 3rd and 4th harmonics our proxy record correlates visually and statistically cross spectral analysis with the atmospheric 14 c record solar proxy and the gisp2 greenland ice sheet project two 18 o record climate proxy showing that solar minima are in phase with drought periods in the northern great plains and cold periods in greenland this spectral similarity together with phase correlation indicates a possible teleconnection of century scale global climate fluctuations through common solar forcing

a 1000 year record of dry conditions in the eastern canadian prairies reconstructed from oxygen and carbon isotope measurements on lake winnipeg sediment organics a short sediment core 162cm covering the period ad 920 1999 was sampled from the south basin of lake winnipeg for a suite of multi proxy analyses leading towards a detailed characterisation of the recent millennial lake environment and hydroclimate of southern manitoba canada information on the frequency and duration of major dry periods in southern manitoba in light of the changes that are likely to occur as a result of an increasingly warming atmosphere is of specific interest in this study intervals of relatively enriched lake sediment cellulose oxygen isotope values d 18 ocellulose were found to occur from ad 1180 to 1230 error range ad 1104 1231 to 1160 1280 1610 1640 error range ad 1571 1634 to 1603 1662 1670 1720 error range ad 1643 1697 to 1692 1738 and 1750 1780 error range ad 1724 1766 to 1756 1794 regional water balance inferred from calculated lake winnipeg water oxygen isotope values d 18 oinf lw suggest that the ratio of lake evaporation to catchment input may have been 25 40 higher during these isotopically distinct periods associated with the enriched d 18 ocellulose intervals are some depleted carbon isotope values associated with more abundantly preserved sediment organic matter d 13 com these suggest reduced microbial oxidation of terrestrially derived organic matter and or subdued lake productivity during periods of minimised input of nutrients from the catchment area with reference to other corroborating evidence it is suggested that the ad 1180 1230 1610 1640 1670 1720 and 1750 1780 intervals represent four distinctly drier periods droughts in southern manitoba canada additionally lower magnitude and duration dry periods may have also occurred from 1320 to 1340 error range ad 1257 1363 1530 1540 error range ad 1490 1565 to 1498 1572 and 1570 1580 error range ad 1531 1599 to 1539 1606 copyright 2009 john wiley sons ltd

solar modulation of north atlantic central water formation at multidecadal timescales during the late holocene article i nfo understanding natural climate variability in the north atlantic region is essential not only to assess the sensitivity of atmosphere ocean climate signal exchange and propagation but also to help distinguish between natural and anthropogenic climate change the north atlantic oscillation is one of the controlling modes in recent variability of atmosphere ocean linkages and ice freshwater fluxes between the polar and north atlantic ocean through these processes the nao influences water mass formation and the strength of the atlantic meridional overturning circulation and thereby variability in ocean heat transport however the impact of the nao as well as other forcing mechanisms on multidecadal timescales such as total solar irradiance on eastern north atlantic central water production central water circulation and climate signal propagation from high to low latitudes in the eastern subpolar and subtropical basins remains uncertain here we use a 1200 yr long benthic foraminiferal mg ca based temperature and oxygen isotope record from a 900 m deep sediment core off northwest africa to show that atmosphere ocean interactions in the eastern subpolar gyre are transferred at central water depth into the eastern boundary of the subtropical gyre further we link the variability of the nao over the past 165 yrs and solar irradiance late holocene and their control on subpolar mode water formation to the multidecadal variability observed at mid depth in the eastern subtropical gyre our results show that eastern north atlantic central waters cooled by up to 0 8 0 7 c and densities decreased by 0 3 0 2 during positive nao years and during minima in solar irradiance during the late holocene the presented records demonstrate the sensitivity of central water formation to enhanced atmospheric forcing and ice freshwater fluxes into the eastern subpolar gyre and the importance of central water circulation for cross gyre climate signal propagation during the late holocene 2011 elsevier b v all rights reserved

a high resolution 11 400 yr diatom record from lake victoria east africa abstract fine interval 30 45 yr sampling of a core from lake victoria s damba channel shows that numerous abrupt changes in the lake s diatom assemblages have occurred in response to climatic fluctuations over the past 11 400 14 c yr four distinct climatic phases bounded by sudden transitions are inferred 1 variably dry 11 400 10 000 yr b p 2 humid 10 000 7200 yr b p 3 more seasonal 7200 2200 yr b p and 4 more arid 2200 0 yr b p with a dry little ice age event 600 200 yr b p the diatom inferred paleoclimatic history for northern lake victoria closely resembles that inferred from a well dated pollen record from pilkington bay spectral analysis of the diatom record reveals strong periodicities including globally distributed 2360 2550 1400 1030 1130 and 500 cal yr cycles repeated rapid shifts between aulacoseira and nitzschia dominated diatom assemblages suggest that post 1960 changes in the lake s phytoplankton communities have had earlier climate driven analogs

record of volcanism since 7000 b c from the gisp2 greenland ice core and implications for the volcano climate system sulfate concentrations from continuous biyearly sampling of the gisp2 greenland ice core provide a record of potential climate forcing volcanism since 7000 b c although 85 percent of the events recorded over the last 2000 years were matched to documented volcanic eruptions only about 30 percent of the events from 1 to 7000 b c were matched to such events several historic eruptions may have been greater sulfur producers than previously thought there are three times as many events from 5000 to 7000 b c as over the last two millennia with sulfate deposition equal to or up to five times that of the largest known historical eruptions this increased volcanism in the early holocene may have contributed to climatic cooling

natural and anthropogenic changes in atmospheric co2 over the last 1000 years from air in antarctic ice and firn a record of atmospheric co2 mixing ratios from 1006 a d to 1978 a d has been produced by analysing the air enclosed in three ice cores from law dome antarctica the enclosed air has unparalleled age resolution and extends into recent decades because of the high rate of snow accumulation at the ice core sites the co2 data overlap with the record from direct atmospheric measurements for up to 20 years the effects of diffusion in the firn on the co2 mixing ratio and age of the ice core air were determined by analyzing air sampled from the surface down to the bubble close off depth the uncertainty of the ice core co2 mixing ratios is 1 2 ppm 1 preindustrial co2 mixing ratios were in the range 275 284 ppm with the lower levels during 1550 1800 a d probably as a result of colder global climate natural co2 variations of this magnitude make it inappropriate to refer to a single preindustrial co2 level major co2 growth occurred over the industrial period except during 1935 1945 a d when co2 mixing ratios stabilized or decreased slightly probably as a result of natural variations of the carbon cycle on a decadal timescale

seasonal variations of glaciochemical isotopic and stratigraphic properties in siple dome antarctica surface snow six snow pit records recovered from siple dome west antarctica during 1994 are used to study seasonal variations in chemical major ion and h 202 isotopic deuterium and physical stratigraphic properties during the 1988 94 period comparison of d measurements and satellite derived brightness temperature for the siple dome area suggests that most seasonal sd maxima occur within 4 weeks of each 1 january several other chemical species h 2o2 non sea salt nss so 4 2 methanesulfonic acid and no3 show coeval peaks with sd together providing an accurate method for identifying summer accumulation sea salt derived species generally peak during winter spring but episodic input is noted throughout some years no reliable seasonal signal is identified in species with continental sources nssca 2 nss mg 2 nh 4 or nsscl visible strata such as large depth hoar layers 5 cm are associated with summer accumulation and its metamorphosis but smaller hoar layers and crusts are more difficult to interpret a multi parameter approach is found to provide the most accurate dating of these snow pit records and is used to determine annual layer thicknesses at each site significant spatial accumulation variability exists on an annual basis but mean accumulation in the sampled 10 km 2 grid for the 1988 94 period is fairly uniform

carbonate microbialites and hardgrounds from manito lake an alkaline hypersaline lake in the northern great plains of canada abstract manito lake is a large perennial na so 4 dominated saline to hypersaline lake located in the northern great plains of western canada significant water level decrease over the past several decades has led to reduction in volume and surface area as well as an increase in salinity the salinity has increased from 10 ppt to about 50 ppt tds this decrease in water level has exposed large areas of nearshore microbialites these organogenic structures range in size from several cm to over a meter and often form large bioherms several meters high they have various external morphologies vary in mineralogical composition and show a variety of internal fabrics from finely laminated to massive in addition to microbiolities and bioherms the littoral zone of manito lake contains a variety of carbonate hardgrounds pavements and cemented clastic sediments dolomite and aragonite are the most common minerals found in these shoreline structures however calcite after ikaite monohydrocalcite magnesian calcite and hydromagnesite are also present the dolomite is nonstoichiometric and calcium rich the magnesian calcite has about 17 mol mgco 3 ams radiocarbon dating of paired organic matter and endogenic carbonate material confirms little or no reservoir affect although there is abundant evidence for modern carbonate mineral precipitation and microbialite formation most of the larger microbialites formed between about 2300 and 1000 cal bp whereas the hardgrounds cements and laminated crusts formed about 1000 500 cal bp

charcoal signatures defined by multivariate analysis of charcoal records from 10 lakes in northwest wisconsin usa article i nfo article history received 16 july 2007 available online 20 november 2010 we show how sedimentary charcoal records from multiple sites within a single landscape can be used to compare fire histories and reveal small scale patterns in fire regimes our objective is to develop strategies for classifying and comparing late holocene charcoal records in midwestern oak and pine dominated sand plain ecosystems where fire regimes include a mix of surface and crown fires using standard techniques for the analysis of charcoal from lake sediments we compiled 1000 to 4000 yr long records of charcoal accumulation and charcoal peak frequencies from 10 small lakes across a sand plain in northwestern wisconsin we used cluster analysis to identify six types of charcoal signatures that differ in their charcoal influx rates amount of grass charcoal and frequency and magnitude of charcoal peaks the charcoal records demonstrate that while fire histories vary among sites there are regional patterns in the occurrence of charcoal signature types that are consistent with expected differences in fire regimes based on regional climate and vegetation reconstructions the fire histories also show periods of regional change in charcoal signatures occurring during times of regional climate changes at 700 1000 and 3500 cal yr bp

a 10 century comparison of prominent bipolar volcanic events in ice cores measurements of key chemical and physical parameters made along continuous and selected long sections of polar ice cores provide reliable past snow accumulation rates and other environmental records the prime accumulation indicators include variations found in the stable isotopes ionic constituents and acidity concentration levels and physical changes in the strata and structure cross correlations of the time series curves resulting from multiparameter analyses of ice cores from antarctica nby89 sp78 bs68 and greenland cr74 d3 81 d3 18c have been made the results permit construction of a bipolar chemical stratigraphy chronology of volcanic events that is coherent with the 18 o and electrical conductivity method data sets and consistent back in time with historically recorded volcanic activity a bihemispheric comparison of high non sea salt so 4 2 peaks shows strong correlation at 1885 1836 1816 1810 1459 1346 1287 1278 1259 1228 and 1168 a d at time of snow deposit the prominent 1259 a d volcanic signal appears to be a significant bipolar ice core index horizon

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

late quaternary eolian sand depositional record for southwestern kansas landscape sensitivity to droughts abstract eolian deposits and landforms are ubiquitous in western kansas particularly south of the arkansas river with the presence of a stabilized dune field stratigraphic studies and associated optical dating reveal a complex depositional history for this dune field spanning the late quaternary the oldest eolian deposits from ca 16 to 12 ka completely or partially bury high terraces with ages of ca 16 and 33 ka it is unknown whether these eolian deposits reflect regional aridity and or a change in sediment availability with the transition of the arkansas river from a single channel to a braided system there is pervasive evidence for episodic eolian erosion and deposition ca 9 8 to 6 3 ka generally coincident with loess deposition on upland surfaces olson c g nettleton w d porter d a brasher b r 1997 middle holocene aeolian activity on the high plains of west central kansas holocene 7 3 255 261 and thus it is inferred to reflect regional aridity sites within the dune field show a sequence of eolian sands and weak buried soils reflecting either dune migration or sand sheet accretion at ca 1490 430 380 320 180 and 70 yr ago which correspond well with continental scale droughts in the tree ring record eolian sand in the arkansas river dune field may be derived principally with variability in fluvial activity and in climate during the late pleistocene to the middle holocene with substantial reactivation of eolian systems during decadal scale drought variability in the past 2000 yr

evaporite minerals of the lower 538 5 m sediments in a long core from the western qaidam basin tibet abstract qaidam basin is a tectonically controlled mesozoic cenozoic depression on the northern margin of the tibetan plateau a 938 5 m long core was drilled in the qahansilatu sub basin in the western qaidam basin with an average core recovery of 95 it revealed alternating salt layers and carbonate clay layers samples were collected at 10 40 cm intervals for mineralogical analysis by xrd and chemical analysis by icp oes the lower 538 5 m sediments are composed of halite gypsum anhydrite gaylussite calcite aragonite ankerite dolomite and an unnamed mineral mg0 92ca0 08co3 3h2o with trace eugsterite the mother brines could be na type na ca type na ca mg type ca na type ca mg na type and trace ca mg na reflux and bacterial activity could be suitable for the formation of dolomite deep burial diagenesis could have played an active role in the formation of ankerite and anhydrate the continuous presence of halite suggested the paleo lake water was highly brackish or saline with high evaporation since 2 77 ma salt layers in the lower 538 5 m long sediments were present from 2221 ka to 2052 ka corresponding to pleistocene salt formation in the qaidam basin as hydrated carbonate minerals both gaylussite and the unnamed mineral are deposited under high precipitation rates with microbial activity gaylussite was deposited from na ca enriched solutions with molar ratios of na ca more than 2 from 691 m 2226 ka to 413 6 m 1222 ka the un named mineral mg0 92ca0 08co3 3h2o was found from 523 4 m 1728 ka to 724 2 m 2308 ka anhydrite could be transformed from gypsum under deep burial from 657 42 m 2052 ka to 867 m 2556 ka alternating salt and clay layers in the lower part of the core recorded arid and relatively wet climatic oscillations and the evolution of brine as a tectonic sub basin tectonic activities could change the local climate during episodes of uplift and subsidence the dominant minerals in the chahansilatu sub basin are similar to those of the other sub basins in the western qaidam basin but have asynchronous evolutionary stages of brine

hydrologic variation in the northern great plains during the last two millennia abstract reconstructions of lake water salinity at decadal resolution for the last 2000 yr are compared among three lakes in north dakota to infer regional patterns of drought the intersite comparisons are used to distinguish local variation in climate or hydrology from regional patterns of change at one lake diatom inferred salinity and lake water mg ca inferred from ostracode shell chemistry are coherent both in terms of direction and magnitude of change indicating that each is a robust technique for reconstructing lake water chemistry the data show that the last 2000 yr have been characterized by frequent shifts between high and low salinity suggesting shifts between dry and moist periods long intervals of high salinity suggest periods of multiple decades when droughts were intense and frequent thus indicating times when drought was more persistent than in the 20th century both the medieval period and little ice age were hydrologically complex and there is no clear evidence to suggest that either interval was coherent or unusual in effective moisture relative to long term patterns differences among the three sites may be attributed to variation in local hydrology and these differences emphasize the need for multiple sites in deriving regional climate interpretations from paleoecological data

mid to late holocene climate driven regime shifts inferred from diatom ostracod and stable isotope records from lake son kol central tian shan kyrgyzstan abstract arid central asia represents a key region for understanding climate variability and interactions in the northern hemisphere patterns and mechanisms of holocene climate change in arid central asia are however only partially understood multi proxy data combining diatom ostracod sedimentological geochemical and stable isotope analyses from a ca 6000 year old lake sediment core from son kol central kyrgyzstan show distinct and repeated changes in species assemblages diatom and ostracod inferred conductivity shifts between meso euhaline and freshwater conditions suggest water balance and regime shifts organism derived data are corroborated by stable isotope mineralogical and geochemical records underlining that son kol was affected by strong lake level fluctuations of several meters the 13ccarb 18ocarb correlation shows repeated switchovers from a closed to an open lake system from 6000 to 3800 and 3250 to 1950 cal yr bp son kol was a closed basin lake with higher conductivities increased nutrient availability and a water level located below the modern outflow son kol became again a hydrologically open lake at 3800 and 1950 cal yr bp comparisons to other local and regional paleoclimate records indicate that these regime shifts were largely controlled by changing intensity and position of the westerlies and the siberian anticyclone that triggered changes in the amount of winter precipitation a strong influence of the westerlies ca 5000 4400 3800 3250 and since 1950 cal yr bp enhanced the amount of precipitation during spring autumn and winter whereas cold and dry winters prevailed during phases with a strong siberian anticyclone and southward shifted westerlies at ca 6000 5000 4400 3800 and 3250 1950 cal yr bp similarities between variations in winter precipitation at son kol and records of the predominant nao mode further suggest a teleconnection between wet dry winter climate in central asia and a positive negative nao mode thus this study identifies climate fluctuations as the main driver for hydrological regime shifts in son kol controlling physicochemical conditions and consequently causing abrupt species assemblage changes this emphasizes the importance of multi proxy approaches to identify triggers thresholds and cascades of aquatic ecosystem transformations

reconstruction of holocene lake level from diatoms chrysophytes and organic matter in a drainage lake from the experimental lakes area northwestern ontario canada abstract a paleolimnological study of lake level changes in lake 239 rawson lake a headwater lake in the experimental lakes area in northwestern ontario indicates large fluctuations have occurred over the holocene analyses are based on diatoms the proportion of chrysophyte scales to diatoms and organic matter content from near shore sediment cores quantitative estimates of lake level are based on a diatom inferred depth model that was developed from surficial sediments collected along several transects in lake 239 declines of 1 3 m occurred during the late holocene whereas declines of at least 8 m occurred during the more arid mid holocene these results provide the first substantive evidence of large declines in lake level in northwestern ontario during the mid holocene conditions during the mid holocene may provide a partial view of future conditions under increasing global temperatures

a 900 year pollen inferred temperature and effective moisture record from varved lake mina west central minnesota usa abstract drought is endemic to the north american great plains causing severe economic consequences however instrumental climate data only exist from ca ad 1890 and limited tree ring paleolimnological archeological and eolian records document the last two millennia to address this lack of monitoring and paleoclimatic data the pollen preserved in the varved sediments of lake mina minnesota on the northeastern border of the great plains were analyzed may and february mean monthly temperatures and annual precipitation minus potential evapotranspiration were reconstructed at a 4 year resolution using a pre settlement pollen climate calibration set the period of the so called little ice age lia ad 1500 1870 was colder than the medieval climate anomaly mca ad 1100 1500 in west central minnesota winter temperatures in the lia declined more than summer ones the pollen record suggests that the lia occurred in three phases an initial cold phase from ad 1505 to ad 1575 a warmer phase and then a very cold phase from ad 1625 to ad 1775 there were severe droughts detected in the lake mina record from ad 1660 to ad 1710 and ad 1300 to ad 1400 suggesting that high resolution pollen records can detect events previously defined from the tree ring records this latter century scale drought is concurrent with the widely reported ad 1250 1400 mega drought which exceeds the severity of 20th century droughts

a 900 yr diatom and chrysophyte record of spring mixing and summer stratification from varved lake mina west central minnesota usa a high resolution independent pollen inferred paleoclimate record and direct algal seasonality data from the actual time of sediment deposition are used to interpret the high resolution diatom and

relative sea level change in greenland during the last 700 yrs and ice sheet response to the little ice age this paper presents new evidence regarding relative sea level rsl changes and vertical land motions at three sites in greenland since 1300 a d a time interval that spans the later part of the medieval climate anomaly mca and the little ice age lia we observe rsl rise at two sites in central west greenland from c 0 80 0 20 m at c 1300 a d to c 0 20 m 0 25 m at c 1600 a d after which rsl slowed and then stabilised at a third site in south greenland we observe rsl rise from c 1 40 0 20 m at c 1400 a d until c 1750 a d after which rsl slowed and was stable during at least the latter part of the 20th century the c 1600 a d rsl slow down seen at the two former sites is surprising because it occurs during the lia when one might expect the ice sheet to be gaining mass and causing rsl to rise we interpret this rsl slowdown to indicate a period of enhanced regional mass loss from central west greenland since c 1600 a d and propose two hypotheses for this loss first a reduction in precipitation during cold and dry conditions and second higher air temperatures and increased peripheral surface melt of the ice sheet from this date onwards the latter hypothesis is compatible with a well established temperature seesaw between western greenland and northern europe and potentially a previously identified shift from a positive to generally more negative nao conditions around 1400 to 1600 a d our study shows how rsl data from greenland can provide constraints on the timing of ice sheet fluctuations in the last millennium and challenges the notion that during cold periods in northern europe the ice sheet in west greenland gained mass

holocene environmental and climatic change in the northern great plains as recorded in the geochemistry of sediments in pickerel lake south dakota the sediments in pickerel lake northeastern south dakota provide a continuous record of climatic and environmental change for the last 12000 yr sediments deposited between 12 and 6 ka radiocarbon show extreme variations in composition oxygen and carbon isotopic composition of bulk carbonate carbon isotopic composition of organic matter and magnetic susceptibility these variations re ect changes in sources of moisture regional vegetation types precipitation evaporation balance ground and surface water in ux water residence time erosion lake productivity water level and water temperature the total carbonate content of late pleistocene sediments steadily increased from 20 at the base of the core to as much as 80 in sediments deposited between 11 and 9 ka by about 8 ka the total carbonate content of the sediments had declined to about 40 where it remained with little variation for the past 8 kyr suggesting relatively stable conditions there are marked increases in values of d13c and d18 oi n bulk carbonate and d13c of organic matter in sediments deposited between 10 and 6 ka as evaporation increased and the vegetation in the watershed changed from forest to prairie this shift toward more 18o enriched carbonate may also re ect a change in source or seasonality of precipitation during this early holocene interval the organic carbon oc content of the sediments remained relatively low 2 3 but then increased rapidly to 4 5 between 7 and 6 ka re ecting the rapid transition to a prairie lake the oc content uctuates slightly between 4 and 6 in sediments deposited over the past 6 kyr like oc and total carbonate most variables measured show little variation in the 13 m of sediment deposited over the past 6 kyr particularly when compared with early holocene variations although the magnetic susceptibility of this upper 13 m of sediment is generally low 10 si units the upper six meters of the section is marked by striking 1 m cycles ca 400 500 yr periodicity in susceptibility these cycles are interpreted as being due to variations in the in ux of eolian detrital clastic material century scale cyclic variations in di erent proxy variables for aridity and eolian activity from sediments deposited over the past 2000 yr in other lakes in the northern great plains as well as in sand dune activity suggest that aridity cycles were the dominant feature of late holocene climate of the northern great plains 2000 elsevier

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

widespread drought episodes in the western great lakes region during the past 2000 years geographic extent and potential mechanisms we compared high resolution reconstructions of peatland water table depth at two raised bogs in the western great lakes region separated by 1000 km the sites included minden bog in southeastern michigan and hole in the bog in north central minnesota our objectives were to 1 determine whether large decadal to multidecadal droughts of the past 2000 yrs were spatially and temporally coherent across the region and 2 assess the underlying mechanisms of widespread droughts in the region we found a strong correlation between bog inferred records of moisture variability in the two regions r 0 53 between 2100 and 600 bp extreme drought events centered on 1850 1800 1650 1000 800 and 700 bp are recorded in both regions many of these drought events were contemporaneous with large droughts already documented at sites far to the west in the great plains and rocky mountains to identify potential modern climate analogues we used empirical orthogonal function eof analysis to reveal spatial modes of variability in drought and precipitation records of north america for the past century the first eofs of annual precipitation and palmer drought severity index pdsi were similar to each other and consistent with a pattern of widespread continental drought correlations between these eofs and annual sea surface temperatures ssts indicate strong relationships to ssts in both the atlantic and pacific basins we hypothesize that widespread droughts between 1000 and 700 bp were related to amplification of a spatial mode of moisture variability associated with an anomalously warm north atlantic a warm mid latitude north pacific and an anomalously cold tropical pacific a more extensive network of hydroclimate records developed using a consistent set of methods and proxies could be used in conjunction with high resolution records of past ssts to test this hypothesis

a 20 000 year record of ocean circulation and climate change from the santa barbara basin much of the evidence for climate driven fluctuations in ocean circulation during the past 20 000 years has come from studies of the north atlantic region1 6 the extent to which such interactions have occurred in other ocean basins and any associated teleconnec tions between basins is poorly understood here we present high resolution palaeoclimate and palaeoceanographic records from a 20 000 year sedimentary sequence from the santa barbara basin on the eastern margin of the north pacific ocean the sequence shows oscillations of the benthic environment between low oxygen conditions laminated sediments during periods of warm climate and higher oxygen conditions non laminated bioturbated sediments during cool intervals age differences between coexisting benthic and planktonic foraminifers indicate climate related changes in the age and source and hence oxygen content of basin bottom waters relatively young bottom waters are associated with the cooler intervals and are considered to reflect high proportions of intermediate waters derived from proximal sources conversely older bottom waters are associated with the warmer intervals and were derived from more distal sources these climate driven variations in ocean circulation appear to be synchronous with the main ocean climate fluctuations in the north atlantic region1 6 suggesting that a tight coupling mechanism operates between the two basins

diatom pollen and chemical evidence of postglacial climatic change at big lake south central british columbia canada abstract postglacial climatic conditions were inferred from cores taken from big lake in southern british columbia low concentrations of nonarboreal pollen and pigments near the base of the core suggest that initial conditions were cool increases in both aquatic and terrestrial production suggest warmer and moister conditions until 8500 cal yr b p hyposaline diatom assemblages increases in nonarboreal pollen and increased concentrations of pigments suggest the onset of arid conditions from 8500 to 7500 cal yr b p slightly less arid conditions are inferred from 7500 until 6660 cal yr b p based on the diatoms small increases and greater variability in biogenic silica and pigments and higher percentages of arboreal pollen at 6600 cal yr b p changes in diatoms pigments biogenic silica and organic matter suggest that big lake became fresh deep and eutrophic until 3600 cal yr b p when water levels and nutrients decreased slightly our paleoclimatic inferences are similar to pollen based studies until 6600 cal yr b p however unlike these studies our multiple lines of evidence from big lake imply large changes in effective moisture since 6000 cal yr b p

spatial variability of climate and land use effects on lakes of the northern great plains evaluation of the effects of climate change and human activities on lakes requires improved understanding of how stressors interact and the degree to which individual sentinel lakes represent broad spatial patterns of ecosystem response to disturbance we surveyed modern water chemistry major ions conductivity salinity lake volume and sediments algal pigments stable isotopes in 21 lakes that surround humboldt lake saskatchewan site of a 2 000 yr climate reconstruction to quantify spatial synchrony s the mean among lake correlation coefficient of prairie lake response to climate variability land use and their interactions whole lake mass balances of total dissolved substances constructed at each site revealed that evaporation of water controlled seasonal changes in salt content only in years with dry summers 2003 leading to widespread spatial coherence of ecosystems s 5 0 78 in contrast variations in hydrologic inputs precipitation groundwater and solute fluxes regulated salt balances of lakes during years with wet summers 2004 2005 and substantially reduced lake synchrony s 5 0 13 0 58 furthermore 25 of sites exhibited increased nitrogen influx as d15n and cyanobacterial production as fossil pigments between ca 1920 and 2003 with particularly strong effects of land use recorded for northeastern sites where evaporative forcing was greatest finally principal component and canonical ordinations with redundancy analysis both explained 50 of the variance in lake sensitivity to climate and land use and revealed that the effects of climate and land use interacted strongly but that the unique effects of each factor remained identifiable in modern lake surveys

interactions of climate and fire at two sites in the northern great plains usa abstract the relationship of fire and climate in explaining the origin and maintenance of the grasslands of the northern great plains has long been of interest i examined the hypothesis that burning was more frequent during wet years using charcoal to reconstruct fire histories near coldwater lake in southcentral nd and rice lake in northcentral nd and published records of ostracod mg ca ratios as a proxy for climate at these same two sites over the past 10 000 years charcoal influx rates for coldwater 0 04 5 68 mm 2 cm 2 year 1 greatly exceeded influx rates for rice 2 cm 2 year 1 both sites showed strong significant periodicities of 1500 1800 800 900 and 130 140 years but charcoal maxima and minima were only similar at 4200 cal years bp charcoal influx during the past 2000 years also revealed strong periodicities of between 310 400 and 140 years at both sites comparison of smoothed charcoal influxes and ostracod mg ca ratios suggests that both proxies are responding to changes in climate when smoothed with a 320 400 year window increases in charcoal influx typically preceded rises in ostracod mg ca ratio by 50 100 years and further smoothing 140 year window of the residuals suggested a variable relationship between mg ca ratios and charcoal influx these results do not clearly support or reject the fuel limitation hypothesis and this lack of clear support may result from a climate driven shifts in the mix of c 3 and c 4 grasses resulting in maximum productivity at intermediate moisture levels b possibly different responses of charcoal and mg ca ratios to summer vs winter precipitation or c ground water driven lags in the response of mg ca ratios to shifts in climate comparison of the results from this study with other studies suggests that links between fire and fuels at finer time scales 0 100 years may be broadly constrained by longer term 500 2000 years patterns of climate in the northern great plains

persistent millennial scale shifts in moisture regimes in western canada during the past six millennia inferences of past climatic conditions from a sedimentary record from big lake british columbia canada over the past 5 500 years show strong millennial scale patterns which oscillate between periods of wet and drier climatic conditions higher frequency decadal to centennial scale fluctuations also occur within the dominant millennial scale patterns these changes in climatic conditions are based on estimates of changes in lake depth and salinity inferred from diatom assemblages in a well dated sediment core after periods of relative stability abrupt shifts in diatom assemblages and inferred climatic conditions occur approximately every 1 220 years the correspondence of these shifts to millennial scale variations in records of glacial expansion recession and ice rafting events in the atlantic suggest that abrupt millennial scale shifts are important to understanding climatic variability in north america during the mid to late holocene unfortunately the spatial patterns and mechanisms behind these large and abrupt swings are poorly understood similar abrupt and prolonged changes in climatic conditions today could pose major societal challenges for many regions

seismological study of lau back arc crust mantle water magmatic differentiation and a compositionally zoned basin abstract on the basis of seismic tomography analyses a structural model of crust formed along the eastern lau spreading center elsc is presented as evidence for a transition from a hydrous type of oceanic crust to a more typical oceanic crust the seismic data indicate that as the spreading center moved away from the active arc the crust thinned from 8 9 km to 7 km the lower crust changed from high p wave velocity values 7 2 7 4 km s to typical values for oceanic crust 7 0 7 2 km s and the upper crustal volcanic layer changed from a thick low velocity layer to a thinner layer with more typical wave speeds the seismic results in combination with other geophysical and geochemical data suggest that crustal formation along the elsc is strongly controlled by the influence of slab water when a spreading center is near the active arc water from the downgoing slab is entrained in the melting zone beneath the ridges where it enhances melting thereafter the water enhances crustal differentiation within sub ridge magma chambers this creates an anomalous hydrous form of oceanic crust with a thick felsic volcanic layer and a mafic ultramafic lower crust features that are not typically observed in crust formed at mid ocean ridges the lau basin has a zoned structure with an abrupt transition from this type of oceanic crust to more typical oceanic crust which resulted from a rapid change in the influence of slab water as the ridge moved away from the arc the abundance and high rate of production of the hydrous crust suggests that such crust may make up a significant proportion of the arc like crust that forms continents

the dilemma of disappearing diatoms incorporating diatom dissolution data into palaeoenvironmental modelling and reconstruction abstract taphonomic issues pose fundamental challenges for quaternary scientists to recover environmental signals from biological proxies and make accurate inferences of past environments the problem of microfossil preservation specifically diatom dissolution remains an important but often overlooked source of error in both qualitative and quantitative reconstructions of key variables from fossil samples especially those using relative abundance data a first step to tackling this complex issue is establishing an objective method of assessing preservation here diatom dissolution that can be applied by different analysts and incorporated into routine counting strategies here we establish a methodology for assessment of diatom dissolution under standard light microscopy lm illustrated with morphological criteria for a range of major diatom valve shapes dissolution data can be applied to numerical models transfer functions from contemporary samples and to fossil material to aid interpretation of stratigraphic profiles and taphonomic pathways of individual taxa using a surface sediment diatom salinity training set from the northern great plains ngp as an example we explore a variety of approaches to include dissolution data in salinity inference models indirectly and directly results show that dissolution data can improve models with apparent dissolution adjusted error rmse up to 15 lower than their unadjusted counterparts internal validation suggests improvements are more modest with bootstrapped prediction errors rmsep up to 10 lower when tested on a short core from devils lake north dakota which has a historical record of salinity dissolution adjusted models infer higher values compared to unadjusted models during peak salinity of the 1930s 1940s dust bowl but nonetheless significantly underestimate peak values site specific factors at devils lake associated with effects of lake level change on taphonomy preservation and re working implied by dissolution data may override model improvements incorporating dissolution dissolution adjusted salinity models are also applied to a 150 year sediment record from spiritwood lake north dakota which suggests that this lake has a damped and lagged response to major regional climate forcing of salinity during the dust bowl at this site dissolution data also suggest different taphonomic behaviour of taxa related to their seasonal patterns of growth and sedimentation thus dissolution data can improve models and aid interpretation of sedimentary profiles as records of limnological ecological and environmental change filtered by taphonomy

a 200 year sulfate record from 16 antarctic ice cores and associations with southern ocean sea ice extent chemistry data from 16 50 115m deep sub annually dated ice cores are used to investigate spatial and temporal concentration variability of sea salt ss so4 2 and excess xs so4 2 over west antarctica and the south pole for the last 200 years low elevation ice core sites in western west antarctica contain higher concentrations of so4 2 as a result of cyclogenesis over the ross ice shelf and proximity to the ross sea polynya linear correlation analysis of 15 west antarctic ice core so4 2 time series demonstrates that at several sites concentrations of ssso4 2 are higher when sea ice extent sie is greater and the inverse for xsso4 concentrations of xsso4 2 from the south pole site east antarctica are associated with sie from the weddell region and west antarctic xsso4 2 concentrations are associated with sie from the bellingshausen amundsen ross region the only notable rise of the last 200 years in xsso4 around 1940 is not related to sie fluctuations and is most likely a result of increased xsso4 2 production in the mid low latitudes and or an increase in transport efficiency from the mid low latitudes to central west antarctica these high resolution records show that the source types and source areas of ssso4 2 and xsso4 2 delivered to eastern and western west antarctica and the south pole differ from site to site but can best be resolved using records from spatial ice core arrays such as the international trans antarctic scientific expedition itase