improving global shear wave traveltime tomography usingthree dimensional ray tracing and iterative inversion a fully non linear approach to global tomography using s wave arrival times has been implemented using 3 d ray tracing with an iterative linearized inversion scheme the starting model for the 3 d inversion was the long wavelength saw12d model derived from inversion of global waveforms the traveltime tomography leads to the introduction of smaller scale structure and a final model in which small scale detail exists and can be resolved with a smooth interconnection through long wavelength structure the two point ray tracing was implemented using a pseudo bending approach for a full spherical 3 d model of the mantle generally the ray paths in the full 3 d model and 1 d reference model are quite close but the inclusion of a more accurate treatment of the rays improves the resolution of wave speed gradients and the positioning of heterogeneity particularly near strong variations in wave speed for example in subduction zones a further advantage of the use of 3 d ray tracing is that it is possible to undertake resolution tests with fewer approximations with the aid of the non linear inversion a number of global s models have been constructed using different assumptions about the character of the model for example solutions can be produced that are designed to introduce minimum differences from a 1 d reference model a variance reduction of 48 per cent was achieved in the inversions with considerable benefit from the inclusion of iterative inversion with 3 d ray tracing and the improved quality of the data set used in this study resolution in the lower part of the mantle has been improved by supplementing the s arrival time data with sks times for the distance range from 84 to 118 the new global s models retain the general features of models derived by one pass linearized inversion with 1 d ray tracing but provide more focused images with a higher perturbation level for the same damping parameters the new models are able to provide a good definition of features revealed by regional tomography using arrival time and waveform data for example the complex slab morphology beneath the tonga and kermadec regions and the sharp boundary between slow and fast uppermost mantle regions beneath western and eastern europe

ray tracing directly in a 3 d ellipsoidal earth model for multiphase global arrivals seismic ray tracing methods are widely used in seismology with most such algorithms being executed in cartesian or spherical coordinate systems however the actual earth is not a perfect sphere but rather an ellipsoid meaning that results calculated in spherical coordinates may be different from the actual situation the general approach is to first calculate traveltimes in a spherical earth model then apply ellipsoidal time corrections to obtain more accurate traveltime data alternatively one may directly conduct the ray tracing in the ellipsoidal earth model without any time corrections in this paper we extend the functional of the multistage irregular shortest path method previously formulated for a spherical earth model to an ellipsoidal earth model in order to trace multiphase global seismic arrivals the results of two models indicate that the proposed algorithm has high computational accuracy which can be further tuned by decreasing the secondary node spacing comparison tests indicate that the traveltime differences between the ellipsoidal and spherical coordinate ray tracing methods cannot be ignored for direct p and s arrivals reflected pcp and scs arrivals and reflected and converted pcs and scp arrivals the traveltime differences te ts computed by the ellipsoidal and spherical ray tracing methods have different distribution patterns being dependent on the source locations however in general these traveltime differences te ts have relatively large negative values near the polar region and positive values near the equatorial region except for sources located near the polar region for ellipsoidal time correction in a specific case the maximum differences between the traveltimes computed by the ellipsoidal coordinate ray tracing method and the ak135 traveltime table after application of ellipsoidal time corrections are less than 0 1 s meanwhile the maximum differences between the traveltimes predicted by the ellipsoidal coordinate ray tracing method and by the spherical coordinate ray tracing method after application of ellipsoidal time corrections are only 0 075 s for the six stated phases these results indicating that ray tracing could be conducted directly in the ellipsoidal earth model

power spectral analysis of auroral occurrence frequency power spectral analyses have been carried out on a data set of swedish visual auroras for the period 1721 to 1943 besides the anticipated 11 year annual and semiannual cycles an unexpected peak near 1 4 years is found the amplitude of this latter peak while smaller than that of the 6 month and 12 month peaks is statistically significant at between the 5 and 1 level and is found consistently in a sequence of spectra covering consecutive overlapping approximately 22 year data segments the semiannual variation becomes extremely weak during the prolonged solar activity minimum at the beginning of the nineteenth century and also shows some lessening during the prolonged solar activity minimum near the beginning of the twentieth century the 1 4 year peak shows a strong modulation of about 65 to 68 years and an additional long term trend of the order of centuries the origin of this peak is unknown

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

precambrian permafrost horizons as indicators of palaeoclimate abstract periglacial phenomena such as polygonal wedge structures and microrelief features commonly preserved in fossil permafrost horizons can provide valuable information on palaeoclimate including indications of mean annual air temperature seasonal temperature range relative precipitation and evidence for long term 104 years oscillations of climate such features are known from early and late proterozoic rocks and indicate mean annual air temperatures as low as 12 to 20 c or lower and strongly seasonal periglacial climates these palaeoclimatic data together with information on palaeolatitude and proximity to palaeo sea level may impose strong constraints on palaeoclimate models for proterozoic glaciation

sunspot periods in the late precambrian glacial climate and solar planetary relations cycles conspicuous in late precambrian 680 myr glacial varves in australia reveal strong climatic periods of about 11 22 145 and 290 yr and a weaker period near 90 yr the 11 22 and 90 yr cycles equate with sunspot periods and the longer cycles with solar and climatic periods as indicated by tree ring studies the varve cycles provide clear evidence of 11 and 22 yr climatic periods in the geological record and may carry important implications for solar physics and planetary science

rapid and early deglaciation in the central brooks range arctic alaska alpine style glaciation was rare in the arctic during the last glaciation because ice sheets occupied most of the glaciated high latitudes due to the tight coupling of alpine glacier fluctuations with climate the geomorphic evidence of such fluctuations in the brooks range alaska usa presents a unique opportunity to study past climate changes in this portion of the arctic we use cosmogenic 10 be exposure dating to directly date last glacial maximum lgm terminal moraines and deglaciation in the central brooks range 10 be ages from moraine boulders indicate that the lgm culminated at ca 21 ka and was followed by substantial retreat upvalley prior to a second moraine building episode culminating at ca 17 ka subsequent rapid deglaciation occurred between ca 16 ka and 15 ka when glaciers receded to within their neoglacial limits initial deglaciation after the lgm was likely caused by ice sheet induced atmospheric circulation changes and increasing insolation brooks range glaciers largely disappeared during heinrich stadial 1 prior to significant warming in the north atlantic region during the bolling allerod but coincident with global co 2 rise glacier fluctuations during the late glacial period if any were restricted to within their neoglacial extents this new chronology suggests that ice sheet modulated atmospheric circulation and global co 2 dominate glacial climate forcings in arctic alaska

the geologic record of climatic change this paper reviews the principal results from paleoclimate studies and includes background material slanted toward climate modelers the inferred temperature history of the last 4 6 billion years indicates major changes in the components of the earth s climate system a secular change in global insolation receipt is due to a 20 30 increase in solar luminosity since the formation of the earth a co2 h2o greenhouse effect may have offset the lower luminosity during early earth history inferred fluctuations of global temperature have occurred over a broad range of time scales on time scales of 106 108 years paleogeographic factors e g continental drift and sea level changes have contributed significantly to temperature changes associated with transitions between nonglacial and glacial states preliminary modeling efforts indicate that additional factors e g co2 changes in atmospheric circulation must also be considered in order to explain the origin of nonglacial climates the origin of polar ice caps may result from ocean circulation changes that were caused by plate tectonic processes fluctuations of ice volume on a time scale of 10 105 years correlate with insolation variations caused by orbital perturbations feedback interactions within the land sea air ice system e g ocean circulation changes and bedrock dynamics have been responsible for a significant modulation of the orbital signal ice ages may be due to orbitally induced temperature changes superimposed on a global cooling of terrestrial origin

dinocyst based reconstructions of sea ice cover concentration during the holocene in the arctic ocean the northern north atlantic ocean and its adjacent seas abstract sea ice cover extent expressed in terms of mean annual concentration was reconstructed from the application of the modern analogue technique to dinocyst assemblages the use of an updated database which includes 1492 sites and 66 taxa yields sea ice concentration estimates with an accuracy of 1 1 10 holocene reconstructions of sea ice cover were made from dinocyst counts in 35 cores of the northern north atlantic and arctic seas in the canadian arctic the results show high sea ice concentration 7 10 with little variations throughout the interval in contrast in arctic areas such as the chukchi sea and the barents sea the reconstructions show large amplitude variations of sea ice cover suggesting millennial type oscillations with a pacing almost opposite in western vs eastern arctic other records show tenuous changes with some regionalism either in trends or sea ice cover variability during the mid holocene and notably at 6 0 5 ka minimum sea ice concentration is recorded in the eastern fram strait northern baffin bay and labrador sea however this minimum cannot be extrapolated at the scale of the arctic and circum arctic the comparison of recent observations and reconstructions suggests larger variations in the arctic sea ice cover during the last decades than throughout the holocene

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

evidence of deposition from individual tides and of tidal cycles from the francis creek shale host rock to the mazon creek biota westphalian d pennsylvanian northeastern illinois abstract a silt clayrock sequence in the francis creek shale pennsylvanian westphalian d of northeastern illinois is characterized by 1 the presence of numerous clayrock paired bands and 2 cyclic variations in thickness of silt between and within these clayrock pairs the sequence is interpreted to result from tidal influence on inner estuarine sediments the clayrocks representing deposition from suspension at times of tidal slack the interbedded silts and fine sands representing deposition from bottom traction during ebb and flood tidal flow the cyclic variations in thickness is interpreted to represent lunar neap spring forcing of tidal range in a dominantly diurnal tidal system the association of tidal characteristics provides a means whereby the rate of sedimentation can be quantified at about 1 0 m yr of compacted sediment during times of active deposition such a high rate of sedimentation would account for the rapid burial and preservation of soft bodied organisms as indicated by the associated mazon creek biota similar rates in similar settings i e tidal estuaries have recently been reported although the depositional setting of many westphalian sequences was favorable to the preservation of tidal features few examples have been recorded it is suggested that evidence of westphalian tides may have been misinterpreted or overlooked in field studies elsewhere

holocene summer temperature reconstruction from sedimentary chlorophyll content with treatment of age uncertainties kurupa lake arctic alaska quantitative records of pre industrial temperature changes are fundamental for understanding long term natural climate variability we used visible reflectance spectroscopy to measure chlorophyll c

evidence for 18 6 year lunar nodal drought in western north america during the past millennium analysis of 38 tree ring chronologies yields evidence for enhanced drought conditions every 18 6 years in the north american interior for the past millennium cross spectra between a drought area index and 26 temperature records confirm the analysis of currie 1979 1981d as regards a lunar nodal mn term in these data discussion of the mn tidal constituent which appreciably modulates amplitude and phase of four main constituents m2 k1 o1 mf is presented and implications for agriculture are surveyed

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

10 be exposure dating of the timing of neoglacial glacier advances in the ecrins pelvoux massif southern french alps alpine glacier variations are known to be reliable proxies of holocene climate here we present a terrestrial cosmogenic nuclide tcn based glacier chronology relying on 24 new be 10 exposure ages which constrain maximum neoglacial positions of four small to mid sized glaciers rateau lautaret bonnepierre and etages in the ecrins pelvoux massif southern french alps glacier advances marked by mainly lateral moraine ridges that are located slightly outboard of the little ice age lia c 1250 1860 ad maximum positions were dated to 4 25 0 44 ka 3 66 0 09 ka 2 09 0 10 ka c 1 31 0 17 ka and to 0 92 0 02 ka the 4 2 ka advance albeit constrained by rather scattered dates is to our knowledge exposure dated here for the first time in the alps it is considered as one of the first major neoglacial advance in the western alps in agreement with other regional paleoclimatological proxies we further review alpine and northern hemisphere mid to high latitude evidence for climate change and glacier activity concomitant with the 4 2 ka event the 2 1 ka advance was not extensively dated in the alps and is thought to represent a prominent advance in early roman times other neoglacial advances dated here match the timing of previously described alpine neoglacial events our results also suggest that a neoglacial maximum occurred at etages glacier 0 9 ka ago i e during the medieval climate anomaly mca c 850 1250 ad at rateau glacier discordant results are thought to reflect exhumation and snow cover of the shortest moraine boulders overall this study highlights the need to combine several sites to develop robust neoglacial glacier chronologies in order to take into account the variability in moraine deposition pattern and landform obliteration and conservation

chronology of holocene glaciation central brooks range alaska the central brooks range was glacierized in the highest north facing cirques during late middle to late holocene neoglacial time this neoglaciation involved at least 5 major cirque glacier expansions of similar magnitude as based on lichenometric mapping of more than 50 glaciers and radiocarbon dates directly associated with 5 moraines initial stabilization of debris covered glaciers took place by early holocene time but evidently no moraines formed during this interval few morainal ridges are preserved that date from the older expansions but they have been lichenometrically dated 20 age reliability to 3 separate intervals 4400 3500 and 2900 yr b p twelve morainal complexes have ridges indicative of glacial advance that lichenometrically date at 1800 500 yr b p relict lichens that are now emerging undisturbed from beneath a receding glacier toe imply that this time was one of prolonged recession in at least some parts of the brooks range however radiocarbon analysis of dead moss at this same site dates a subsequent neoglacial advance across the cirque floor at 1120 180 yr b p our data suggest that for the past 1 100 yr cirque glaciers have been continuously in more extended positions than they are today during this glaciologically favorable interval the last two major advances occurred at 800 150 and 390 90 yr b p a d 1410 1600 glaciers across the central brooks range stayed close to their maxima until a d 1640 1750 historical photographs and lichenometry show that retreat was most rapid after a d 1870 and decelerated after the mid 1900s recession from this most recent neoglacial maximum has amounted to 150 700 m and continues at present the cirque glacier advances were accompanied by equilibrium line altitude ela depressions of 100 to 200 m below levels maintained in the late 1970s environmental lapse rate estimates and 1977 1981 glaciologic meteorologic measurements suggest that summer air temperatures accompanying neoglacial maxima were respectively 1 c or 3 to 4 c cooler than those of the late 1970s across the central brooks range the neoglacial maxima elas rise northeastward and northward from 2 to 5 m km 1

gypsum a joint tomographic model of mantle density and seismic wave speeds 1 gypsum is a 3 d model of mantle shear wave s speeds compressional wave p speeds and density the model is developed through simultaneous inversion of seismic body wave travel times p and s and geodynamic observations while using realistic mineral physics parameters linking wave speeds and density geodynamic observations include the global free air gravity field divergence of the tectonic plates dynamic topography of the free surface and the flow induced excess ellipticity of the core mantle boundary gypsum is built with the philosophy that heterogeneity that most closely resembles thermal variations is the simplest possible solution models of the density field from earth s free oscillations have provided great insight into the density configuration of the mantle but are limited to very long wavelength solutions alternatively scaling higher resolution seismic images to obtain density anomalies generates density fields that do not satisfy geodynamic observations the current study provides a 3 d density model for the mantle that directly satisfies geodynamic and seismic observations through a joint seismic geodynamic inversion process notable density field observations include high density piles at the base of superplume structures supporting the general results of past normal mode studies however we find that these features are more localized and have lower amplitude than past studies would suggest when we consider both fast and slow seismic anomalies in gypsum we find that p and s wave speeds are strongly correlated throughout the mantle however we find a low correlation of fast s wave zones in the deep mantle 1500 km depth with the corresponding p wave anomalies suggesting a systematic divergence from simplified thermal effects in ancient subducted slab anomalies the cratonic lithosphere and d regions are shown to have strong compositional signatures however we argue that temperature variations are the primary cause of p wave speed s wave speed and density anomalies throughout most of the mantle

climate record in varved sediments of the eocene green river formation three time series of varves from the eocene green river formation in colorado consisting of 1469 1869 and 4158 annual thickness measurements were analyzed by a variety of statistical techniques to characterize climate variations the analyses reveal that the time series are lognormal after adjustment for a trend moreover the analyses suggest that the time series are generally nonstationary and are noiselike in structure fourier spectra and maximum entropy spectra of the time series are reddish indicating the importance of nonoscillatory changes in varve thickness at time scales ranging from tens to hundreds of years there are no statistically significant peaks in fourier spectra for the entire time series however one segment of one of the three time series displays statistically significant peaks corresponding to periods of 10 8 and 5 4 years overall the time series are suggestive of a nonperiodic and weakly predictive climate during the time periods corresponding to deposition of the varves

the lunar orbit in the late precambrian and the elatina sandstone laminae the discovery of layered sediments of the elatina formation of south australia has attracted intense speculation regarding their origin1 2 the formation 10 m layer of graded sandstone consists of a pattern of light bands of periodically varying thickness3 a sequence of dark bands separates on average 11 6 light bands the spacing of these dark bands exhibits a rich and complex spectrum previously the periodicity has been attributed to the sunspot cycle and unassigned but putative solar periods prompted by a letter from g e williams summarizing new field data from the adelaide region south australia which imply that the elatina varves may be tidal we re examined the dark band spectrum and propose a luni solar tidal interaction model as the source of the laminae

10be ages of late pleistocene deglaciation and neoglaciation in the north central brooks range arctic alaska we present a chronology of late pleistocene deglaciation and neoglaciation for two valleys in the north central brooks range alaska using cosmogenic 10be exposure dating the two valleys show evidence of ice retreat from the northern range front before 16 15 ka and into individual cirques by 14 ka there is no evidence for a standstill or re advance during the lateglacial period indicating that a glacier advance during the younger dryas if any was less extensive than during the neoglaciation the maximum glacier expansion during the neoglacial is delimited by moraines in two cirques separated by about 200 km and dated to 4 6 0 5 and 2 7 0 2 cal ka bp both moraine ages agree with previously published lichen inferred ages and confirm that glaciers in the brooks range experienced multiple advances of similar magnitude throughout the late holocene the similar extent of glaciers during the middle holocene and the little ice age may imply that the effect of decreasing summer insolation was surpassed by increasing aridity to limit glacier growth as neoglaciation progressed copyright 2012 john wiley sons ltd

rapid sea level rise in the aftermath of a neoproterozoic snowball earth earth s most severe climate changes occurred during global scale snowball earth glaciations which profoundly altered the planet s atmosphere oceans and biosphere extreme rates of glacioeustatic sea level rise are predicted by the snowball earth hypothesis but supporting geologic evidence has been lacking we use paleohydraulic analysis of wave ripples and tidal laminae in the elatina formation australia deposited after the marinoan glaciation 635 million years ago to show that water depths of 9 to 16 meters remained nearly constant for 100 years throughout 27 meters of sediment accumulation this accumulation rate was too great to have been accommodated by subsidence and instead indicates an extraordinarily rapid rate of sea level rise 0 2 to 0 27 meters per year our results substantiate a fundamental prediction of snowball earth models of rapid deglaciation during the early transition to a supergreenhouse climate

holocene climatic change in the alaskan arctic as inferred from oxygen isotope and lake sediment analyses at wahoo lake despite recent progress in understanding high latitude climate variability paleoclimate records are scarce from the alaskan arctic we conducted isotopic and sediment analyses at wahoo lake to infer holocene climate variability in northeastern alaska water 18o and d values from the lake and its inlet outlet streams suggest that winter precipitation dominates modern water inputs and that evaporation has limited influence on the lake s hydrological budget the isotopic composition of pisidium exhibits marked variations during the past 11 500 years with 18o ranging between 18 7 and 16 2 and 13c between 7 1 and 2 3 vienna pee dee belemnite vpdb elevated 18o and sediment composition from 11 5 to 8 9 kcal bp suggest evaporative 18o enrichment and arid conditions rising lake levels are evidenced by the disappearance of pisidium and a transition to low carbonate gyttja ca 6 3 kcal bp and by the onset of sediment deposition on an adjacent shelf by 5 3 kcal bp these changes coincided with

a 16 000 yr long sedimentary sequence from lakes peters and schrader neruokpuk lakes northeastern brooks range alaska sediments that accumulate in high latitude lakes serve as valuable environmental archives of changing conditions in a region currently undergoing rapid change a previously unexplored sedimentary sequence reaching back 16 000 years from lakes peters and schrader neruokpuk lakes in the northeastern brooks range 69 n alaska shows distinct changes in accumulation rates and biophysical properties including bulk density bd organic matter om content and grain size distribution at five widely distributed core sites the oldest sediments contain little om and accumulated rapidly as glaciers retreated around 15 ka om peaked between 12 and 10 ka along with northern hemisphere summer insolation bd increased and om decreased until around 5 ka possibly reflecting a decrease in river transported terrestrial om from 5 2 ka om consistently increased suggesting a rise in river discharge or a rise in summer temperatures which led to higher productivity or both after 2 ka sediments increased in bd and decreased in om suggesting glacier growth evidence for glacier expansion late during the little ice age is weak but increased sedimentation rates may reflect glacier retreat during the last century this study provides a framework for future paleoenvironmental research of a rare archive in a relatively pristine arctic setting

joint bulk sound and shear tomography for western pacific subduction zones abstract detailed regional body wave tomographic inversion of the western pacific region has been performed using p and s travel times from common sources and receivers with a joint inversion in terms of bulk sound and shear wave speed variations in the mantle this technique allows the separation of the influence of bulk and shear moduli and hence a more direct comparison with mineral physics information the study region is parameterized with cells of side 0 5 to 2 and 19 layers to a depth of 1500 km while the rest of the mantle was parameterized with 5 5 cells with 16 layers between the surface and the core mantle boundary a simultaneous inversion is made for regional and global structures to minimize the influence of surrounding structures on the regional image a nested iterative inversion scheme is employed with local linearization and three dimensional ray tracing through the successive model updates the results of the regional tomographic inversion reveal the penetration of a subducted slab below the 660 km discontinuity at the kurile kamchatka trench while flattening of slabs above this depth is observed in the japan and izu bonin subduction zones on both the bulk sound and shear wave speed images the penetration of a subducted slab down to a depth of at least 1200 km is seen below the southern part of the bonin trench mariana philippine and java subduction zones fast shear wave speed perturbations associated with the subducted slabs down to the 410 km transition zone are larger than the comparable bulk sound perturbations for all these subduction zones except the philippines the bulk sound signature for the subducted slab is more pronounced than for shear in the philippines talaud new guinea solomon and tonga subduction zones where penetration of the slab into the middle mantle is observed variation in the amplitude ratio between bulk sound and shear wave speed anomalies correlates well with the subduction parameters of the descending slab slabs younger than 90 ma at the trench show bulk sound dominance in the upper mantle while older slabs have a stronger shear wave speed signature spreading of the fast shear wave speed zone between 800 and 1000 km is observed in the areas of deep subducted slab penetration but has no comparable expression in the bulk sound images this high velocity feature may reflect physical or chemical disequilibria introduced to the lower mantle by subducted slabs

holocene climate changes in eastern beringia nw north america a systematic review of multi proxy evidence abstract reconstructing climates of the past relies on a variety of evidence from a large number of sites to capture the varied features of climate and the spatial heterogeneity of climate change this review summarizes available information from diverse holocene paleoenvironmental records across eastern beringia alaska westernmost canada and adjacent seas and it quantifies the primary trends of temperature and moisture sensitive records based in part on midges pollen and biogeochemical indicators compiled in the recently published arctic holocene database and updated here to v2 1 the composite time series from these proxy records are compared with new summaries of mountain glacier and lake level fluctuations terrestrial water isotope records sea ice and sea surface temperature analyses and peatland and thaw lake initiation frequencies to clarify multi centennial to millennial scale trends in holocene climate change to focus the synthesis the paleo data are used to frame specific questions that can be addressed with simulations by earth system models to investigate the causes and dynamics of past and future climate change this systematic review shows that during the early holocene 11 7 8 2 ka 1 ka 1000 cal yr bp rather than a prominent thermal maximum as suggested previously temperatures were highly variable at times both higher and lower than present approximate mid 20th century average with no clear spatial pattern composited pollen midge and other proxy records average out the variability and show the overall lowest summer and mean annual temperatures across the study region during the earliest holocene followed by warming over the early holocene the sparse data available on early holocene glaciation show that glaciers in southern alaska were as extensive then as they were during the late holocene early holocene lake levels were low in interior alaska but moisture indicators show pronounced differences across the region the highest frequency of both peatland and thaw lake initiation ages also occurred during the early holocene during the middle holocene 8 2 4 2 ka glaciers retreated as the regional average temperature increased to a maximum between 7 and 5 ka as reflected in most proxy types following the middle holocene thermal maximum temperatures decreased starting between 4 and 3 ka signaling the onset of neoglacial cooling glaciers in the brooks and alaska ranges advanced to their maximum holocene extent as lakes generally rose to modern levels temperature differences for averaged 500 year time steps typically ranged by 1 2 c for individual records in the arctic holocene database with a transition to a cooler late holocene that was neither abrupt nor spatially coherent the longest and highest resolution terrestrial water isotope records previously interpreted to represent changes in the aleutian low pressure system around this time are here shown to be largely contradictory furthermore there are too few records with sufficient resolution to identify sub centennial scale climate anomalies such as the 8 2 ka event the review concludes by suggesting some priorities for future paleoclimate research in the region

a 10 000 year record of arctic ocean sea ice variability view from the beach we present a sea ice record from northern greenland covering the past 10 000 years multiyear sea ice reached a minimum between 8500 and 6000 years ago when the limit of year round sea ice at the coast of greenland was located 1000 kilometers to the north of its present position the subsequent increase in multiyear sea ice culminated during the past 2500 years and is linked to an increase in ice export from the western arctic and higher variability of ice drift routes when the ice was at its minimum in northern greenland it greatly increased at ellesmere island to the west the lack of uniformity in past sea ice changes which is probably related to large scale atmospheric anomalies such as the arctic oscillation is not well reproduced in models this needs to be further explored as it is likely to have an impact on predictions of future sea ice distribution

holocene dynamics of the arctic s largest ice shelf ice shelves in the arctic lost more than 90 of their total surface area during the 20th century and are continuing to disintegrate rapidly the significance of these changes however is obscured by the poorly constrained ontogeny of arctic ice shelves here we use the sedimentary record behind the largest remaining ice shelf in the arctic the ward hunt ice shelf ellesmere island canada to establish a long term context in which to evaluate recent ice shelf deterioration multiproxy analysis of sediment cores revealed pronounced biological and geochemical changes in disraeli fiord in response to the formation of the ward hunt ice shelf and its fluctuations through time our results show that the ice shelf was absent during the early holocene and formed 4 000 years ago in response to climate cooling paleoecological data then indicate that the ward hunt ice shelf remained stable for almost three millennia before a major fracturing event that occurred 1 400 years ago after reformation 800 years ago freshwater was a constant feature of disraeli fiord until the catastrophic drainage of its epishelf lake in the early 21st century

late pleistocene mountain glacier response to north atlantic climate change in southwest ireland stephan harrison neil glasser ed anderson susan ivy ochs peter w kubik 2010 late pleistocene mountain glacier response to north atlantic climate change in southwest ireland quaternary science reviews 29 27 28 3948 3955

cyclic variation and solar forcing of holocene climate in the alaskan subarctic high resolution analyses of lake sediment from southwestern alaska reveal cyclic variations in climate and ecosystems during the holocene these variations occurred with periodicities similar to those of solar activity and appear to be coherent with time series of the cosmogenic nuclides 14c and 10be as well as north atlantic drift ice our results imply that small variations in solar irradiance induced pronounced cyclic changes in northern high latitude environments they also provide evidence that centennial scale shifts in the holocene climate were similar between the subpolar regions of the north atlantic and north pacific possibly because of sun ocean climate linkages

holocene history of the great kobuk sand dunes northwestern alaska abstract located just north of the arctic circle the great kobuk sand dunes gksd are an inland dune field that is closely surrounded by boreal forest the history of the gksd tells us about changes in aridity a climatic parameter whose history is poorly understood at high latitudes vegetated dunes in several states of geomorphic preservation surround the active dune field today evidencing a complex history of holocene activity small lakes in the forest bordering the dunes accumulate wind blown sand we use 14 c dated lake sediment cores to reconstruct a continuous history of sand influx over the last 8000 yr the validity of this record is supported by limiting ages obtained from stratigraphic sections within the dune field the extent of the gksd underwent a fluctuating shrinkage coincident with neoglaciation this downsizing trend was interrupted by periods of increased sand deposition into lakes occurring 4800 4200 3300 2600 1300 700 and 300 100 calendar years ago aridity in the kobuk valley during the holocene probably was controlled by the frequency of north pacific storms entering the region in late summer our results describe the first continuous history of changing moisture balance for central beringia during the holocene and comprise a baseline against which future records of climatic and ecological change in this region can be compared

holocene climate inferred from oxygen isotope ratios in lake sediments central brooks range alaska analyses of sediment cores from two lakes in the central brooks range provide temperature and moisture balance information for the past 8500 cal yr at century scale resolution two methods of oxygen isotope analysis are used to reconstruct past changes in the effective moisture precipitation minus evaporation and temperature effective moisture is inferred from oxygen isotope ratios in sediment cellulose from meli lake area 0 13 km 2 depth 19 4 m the lake has a low watershed to lake area ratio 7 and significant evaporation relative to input summer temperature shifts are based on oxygen isotope analyses of endogenic calcite from tangled up lake area 0 25 km 2 depth 3 5 m this basin has a larger watershed to lake area ratio 91 and less evaporation relative to input sediment oxygen isotope analyses from the two sites indicate generally more arid conditions than present prior to 6000 cal yr b p subsequently the region became increasingly wet temperature variability is recorded minimally at centennial scale resolution with values that are generally cool for the past 6700 cal yr the timing and direction of climate variability indicated by the oxygen isotope time series from meli and tangled up lakes are consistent with previously established late holocene glacier advances at 5000 cal yr b p in the central brooks range and high lake levels at birch lake since 5500 cal yr b p this unique use of oxygen isotopes reveals both moisture balance and temperature histories at previously undetected high resolution temporal scales for northern alaska during the middle to late holocene c 2001 university of washington

mid proterozoic dolomitic varves and microcycles from the mcarthur basin northern australia abstract the middle proterozoic 1 75 ga wollogorang formation of the unmetamorphosed intracratonic mcarthur basin of northern australia contains a dolomitic black shale facies that has distinctive millimetre and sub millimetre scale bedding laminations and microcycles of alternating organic rich mudstone and organic poor dolostone the microcycles are between 1 and 20 mm thick and their internal laminations are arranged rhythmically into a lower subdivision of mainly dolostone with subordinate mudstone laminae and an upper subdivision of mainly mudstone with thin dolostone laminae the microcycles vary systematically in both thickness and character the thinnest laminations visible around 0 2 mm thick are interpreted as chemical varves the microcycles represent bundles of varves deposited over longer periods over these longer periods each microcycle shows a gradual upward decrease in the amount of dolomite precipitated and or preserved this is related in part to original climatic controls in the environment of deposition and also to a secondary but pre lithification process of dissolution similar to that invoked by trendall 1972 1973 for the formation of mesobands in the bif s of the hamersley basin western australia simple statistical calculations of varve thickness variations on about 100 microcycles from a 1 5 m long core indicate the presence of 7 to 11 year and 300 year periods these periods are comparable with those published by williams 1981 for glaciogenic varves from late proterozoic 680 ma rocks in south australia he relates the former to sunspot cycles and the later to longer term climate periods as evident from tree ring studies although nowhere near as conspicuous as the sunspot cycles described by williams 1981 or the double sunspot cycles of trendall 1972 from the latest and earliest proterozoic respectively these microcycles from the middle proterozoic may be due to similar sunspot related processes and therefore add support to their suggestions for long term stability of solar processes

permafrost evidence for severe winter cooling during the younger dryas in northern alaska 1 the younger dryas cold event a rapid reversion to glacial climate conditions at the pleistocene holocene transition has generally been attributed to the release of meltwater from the laurentide ice sheet to the north atlantic or arctic oceans the reaction of the north pacific region to this shutdown of the thermohaline circulation in the north atlantic during younger dryas is little understood in this paper we present the first radiocarbon dated centennial scale stable water isotope record from permafrost in northern alaska this late glacial winter climate reconstruction from barrow ice wedges demonstrates the existence of a younger dryas cold event formerly believed to be reduced or absent in this area our stable isotope data display a gradual change of the atmospheric moisture source conditions during the younger dryas likely associated with the successive opening of the bering strait

a millennial scale record of arctic ocean sea ice variability and the demise of the ellesmere island ice shelves 1 sea ice ice shelves at the apex of north america 80 n constitute the oldest sea ice in the northern hemisphere we document the establishment and subsequent stability of the ward hunt ice shelf and multiyear landfast sea ice in adjacent fiords using 69 radiocarbon dates obtained on holocene driftwood deposited prior to coastal blockage these dates 47 of which are new record a hiatus in driftwood deposition beginning 5500 cal yr bp marking the inception of widespread multiyear landfast sea ice across northern ellesmere island this chronology together with historical observations of ice shelf breakup 1950 to present provides the only millennial scale record of arctic ocean sea ice variability to which the past three decades of satellite surveillance can be compared removal of the remaining ice shelves would be unprecedented in the last 5500 years this highlights the impact of ongoing 20th and 21st century climate warming that continues to break up the remaining ice shelves and soon may cause historically ice filled fiords nearby to open seasonally

lacustrine varve formation through time abstract studies using sediment traps in lakes reveal that the seasonal flux of sediment regulates both the composition and timing of deposition of materials that reach the bottoms of lakes if the bottom waters of a lake are partly or totally anoxic the seasonally deposited materials are preserved as annual groupings of laminae varves common components that form individual laminae consist of allochthonous clastic material derived from the drainage basin precipitated carbonate minerals diatom frustules iron rich and manganese rich flocs autochthonous organic detritus and autochthonous and allochthonous materials resuspended from the bottom the style of varving has changed over geologic time reflecting changes in biologic evolution and types of materials available precipitated iron rich laminations were common in the middle precambrian graded sets of clastic organic laminations persisted through the precambrian prior to the evolution of bioturbating benthic organisms glaciolacustrine varves appear to have retained their distinctive character through time carbonate rich varves occurred sporadically in the precambrian and phanerozoic with the exception of diatoms major components of modern lacustrine varves were present through the paleozoic and mesozoic and yet varves are rare in strata of these ages and may have accumulated in marine to brackish water environments diatoms were introduced into lacustrine systems in early tertiary time and are common components of varves from then on diatom laminae combined with a greater chance for geologic preservation of younger lake deposits have increased the number of geologically young occurrences of varved sediments however seasonal associations of modern varve components and the processes they represent are present in ancient deposits and provide clues to the interpretation of ancient environments

the alaskan arctic frontal zone forcing by orography coastal contrast and the boreal forest abstract experiments have been conducted with a regional climate model to indicate the conditions required to generate preferred regions of frontal activity in the alaskan region several objective methods of frontal identification were first investigated it was found that the vertical component of relative vorticity a thermal front parameter t850 n where t850 is the 850 hpa temperature and n is a unit vector in the direction of the 850 hpa temperature gradient and a parameter derived from the q vector as a measure of vertical motionwere useful in combination to determine the occurrence of fronts the preferred locations for frontal activity were located to the southern side of the eastern brooks range and over the alaska range these diagnostics were then used to characterize frontal frequency in a series of experiments removing topographic and vegetation contrasts it was found that the removal of the treeline contrast and its associated heating gradient had a small effect on frontal frequ

solar cycles recorded in carboniferous glacimarine rhythmites western argentina relationships between climate and sedimentary environment abstract a carboniferous glacial episode has been widely known in the western part of argentina since ca 1910 in some sections it is associated with rhythmic sedimentation one of those rhythmite successions is interpreted as composed by varves annually deposited sedimentary cycles accumulated in a glacimarine environment it is further analyzed as a time series searching for cyclic signals several localities show varve like successions but only a few of them are not disturbed by reworking of bottom currents the rio francia section san juan argentina is well suited for varve counting as no current produced structures are recognized through the rhythmite interval and outcrops are good enough for counting directly in the field five varve types which show differences mainly in thickness grain size and dropstone frequency are recognized internally they are mostly symmetric distinguishing them from traditional normally graded varves a feature that has also been observed in holocene glacimarine varves the different varve types indicate varying positions with respect to the ice front because of disturbances or lack of well defined textural or colour contrasts only the three varve types with intermediate thicknesses were counted three units including 1028 690 and 465 varves were measured and then analyzed as time series in order to detect cyclic signals some differences in the quality of the cyclic signals can be seen in spectral analyses depending on the main type of the varve that composes the unit however a peak around 12 yrs can be seen in most of the spectra and is interpreted as the sunspot cycle record a secondary peak near 24 26 yr also may be related to this phenomenon hale cycle four major cycles recognized in the field but not in the spectra that represent major glacial advances and retreats can also be related to a solar periodicity of 2500 3000 yr the recognition of sunspot cycles in these carboniferous varves is not unusual as they are found in other ancient varve successions random processes typified by the distribution of dropstones played an important role in disturbing the climatic signal in this sedimentary environment the random process of sediment transport and deposition by ice rafting disturbs the cyclic climatic signal mainly in proximal varve successions increased water motion and distance to the source of sediment toward the open sea tend to produce more homogeneous very fine grained distal varves eventually causing the loss of varve like appearance the remaining varves located at intermediate positions with respect to the glacio related sediment source seem to be the best suited for recording climatic cyclicity as shown by spectral analyses

multiple thermo erosional episodes during the past six millennia implications for the response of arctic permafrost to climate change anthropogenic warming may promote rapid permafrost thaw in the arctic and alter the global carbon cycle although several studies suggest increased thermo erosion as a result of recent warming a long term context is necessary to assess the linkages of thermokarst processes with climate variability we analyzed sediment cores from two lakes on the alaskan north slope usa one with lake ne14 and one without perch lake watershed thermo erosion distinct geochemical and lithological characteristics provide evidence for sedimentary input from carbonate rich permafrost soils associated with past retrogressive thaw slumping at lake ne14 but not at perch lake these characteristics include increases in ca sr ca k carbonate feldspar clay minerals percent caco3 and 13c and decreases in 87sr 86sr at least ten episodes of thermo erosion occurred over the past 6000 yr at lake ne14 most of these episodes coincided with periods of elevated summer temperatures but moisture variation and geomorphic factors likely played a role in driving their occurrence our results suggest that positive feedbacks facilitate reactivation of thermo erosion in ice rich terrain adding to the growing body of evidence that these arctic landscapes are unstable in a changing climate

multiple independent records of local glacier variability on nuussuaq west greenland during the holocene abstract the sensitivity of mountain glaciers to small scale climate fluctuations makes records of their past extent among the best proxies for identifying spatio temporal climate variability here we build on the few existing holocene records of local glacier change in greenland by using three independent geochronological methods proglacial lake sediment analysis cosmogenic 10be surface exposure dating and in situ 14c burial modeling to reconstruct continuous records of holocene glacier variability on nuussuaq west greenland 10be ages of perched boulders indicate eastern nuussuaq was deglaciated between 11 0 and 10 5 ka radiocarbon dated sediments from two lakes on nuussuaq contain mineral rich layers between 9 6 and 9 0 and 8 7 8 0 cal ka bp that may be correlative with nearby ice sheet moraines deposited in the early holocene multiple proxies for glacier size indicate frequent high amplitude glacier fluctuations superimposed on net glacier growth during the late holocene with significant ice expansion phases at 3 7 ka 2 8 ka and throughout the past 2 ka mean 10be ages from five nested moraine crests confirm that local glacier extents on nuussuaq culminated during both the little ice age 1470 c e n 3 and 1750 c e n 3 and the preceding centuries 520 1320 c e n 11 results reveal that local glaciers on nuussuaq episodically advanced and retreated at centennial timescales throughout the holocene most likely in response to regional climate changes in west greenland superimposed on the progressive insolation driven cooling trend in the northern hemisphere our new 10be moraine chronologies coupled with other glacier size proxies corroborate an emerging pattern of significant summer cooling and glacier expansion in the centuries prior to the little ice age in the arctic

century scale solar variability and alaskan temperature change over the past millennium 1 correlation of geologic histories from 130 alaskan glaciers with a record of solar variation suggests that multi decadal to century scale temperature variations in the north pacific and arctic sectors have been influenced by solar forcing over the past thousand years mountain glacier fluctuations are primarily a record of summer cooling and the composite glacial history from three climatic regions across alaska shows ice expansions approximately every 200 years compatible with a solar mode of variability the modulating effects of the cold phases of the pacific decadal oscillation and the arctic oscillation may when in phase with decreased solar activity serve to amplify cooling forcing glacier advance

climatic cycles recorded in glacially influenced rhythmites of the gowganda formation huronian supergroup abstract the gowganda formation of the 2 45 2 2 ga huronian supergroup contains glacially induced varve like rhythmites that potentially preserve a detailed record of climatic conditions during the paleoproterozoic era four rhythmic couplet thickness records were measured at two outcrops near wharncliffe ontario for the purpose of time series analysis the couplets which range from 1 to 32 mm thick are composed of alternating layers of siltstone and claystone time series analysis of the couplet thickness records using the mtm toolkit of mann and lees 1996 consistently revealed periodicities in the range of 2 2 2 9 couplets per cycle which is consistent with climatic cycles such as the quasi biennial oscillation qbo and the el nino southern oscillation enso observed in modern times this periodicity suggests that the rhythmic couplets represent annual deposits i e varves evidence for the presence of cycles at 3 0 4 9 couplets 6 6 6 9 couplets 8 8 9 2 couplets 22 8 couplets and 30 1 31 0 couplets were also observed in some couplet thickness records however the presence of these longer term cycles was inconsistent from site to site

numerical analyses of a multi proxy data set from a distal glacier fed lake sorsendalsvatn western norway abstract here we present a holocene record of glacier variability as documented through physical sediment properties analysed on sediments from the distal glacier fed lake nedre nedre lower sorsendalsvatn 918 m a s l located 35 km inland from the coast in western norway we emphasise comparing different sediment parameters by means of statistical methods as well as integrating chronological uncertainties along with uncertainties of reconstructed glacier variability a multi proxy data set consisting of sedimentological physical and geochemical data shows one main process as extracted by means of principal component analysis 88 of the variance explained by the first pc driving sediment variability in nedre sorsendalsvatn the common signal extracted from the sediment data is indicative of glacial activity in the catchment and is interpreted to vary in concert with the changing glacier equilibrium line altitude the reconstruction of former glacier activity is in accordance with glacier variability reconstructed from other sites in western norway including the termination of the deglaciation at approximately 10 000 cal yr bp the 8 2 ka bp finse event the holocene thermal optimum between 8000 and 5500 cal yr bp and the onset of the neoglacial at 5500 cal yr bp the largest glacial extent during the neoglacial time period took place during the little ice age the combined radiocarbon chronologies from three different sediment cores provide insight into the duration of the 8 2 ka event in the terrestrial system the maximum glacier activity at approximately 8 2 cal bp is the culmination of a glacier advance that began around 9 cal bp and accelerated at 8 4 cal bp the glacier advance ended abruptly at 8 0 cal bp

glacier fluctuations during the past 2000 years abstract a global compilation of glacier advances and retreats for the past two millennia grouped by 17 regions excluding antarctica highlights the nature of glacier fluctuations during the late holocene the dataset includes 275 time series of glacier fluctuations based on historical tree ring lake sediment radiocarbon and terrestrial cosmogenic nuclide data the most detailed and reliable series for individual glaciers and regional compilations are compared with summer temperature and when available winter precipitation reconstructions the most important parameters for glacier mass balance in many cases major glacier advances correlate with multi decadal periods of decreased summer temperature in a few cases such as in arctic alaska and western canada some glacier advances occurred during relatively warm wet times the timing and scale of glacier fluctuations over the past two millennia varies greatly from region to region however the number of glacier advances shows a clear pattern for the high mid and low latitudes and hence points to common forcing factors acting at the global scale globally during the first millennium ce glaciers were smaller than between the advances in 13th to early 20th centuries ce the precise extent of glacier retreat in the first millennium is not well defined however the most conservative estimates indicate that during the 1st and 2nd centuries in some regions glaciers were smaller than at the end of 20th early 21st centuries other periods of glacier retreat are identified regionally during the 5th and 8th centuries in the european alps in the 3rd 6th and 9th centuries in norway during the 10th 13th centuries in southern alaska and in the 18th century in spitsbergen however no single period of common global glacier retreat of centennial duration except for the past century has yet been identified in contrast the view that the little ice age was a period of global glacier expansion beginning in the 13th century or earlier and reaching a maximum in 17th 19th centuries is supported by our data the pattern of glacier variations in the past two millennia corresponds with cooling in reconstructed temperature records at the continental and hemispheric scales the number of glacier advances also broadly matches periods showing high volcanic activity and low solar irradiance over the past two millennia although the resolution of most glacier chronologies is not enough for robust statistical correlations glacier retreat in the past 100 150 years corresponds to the anthropogenic global temperature increase many questions concerning the relative strength of forcing factors that drove glacier variations in the past 2 ka still remain

crust and upper mantle p wave velocity structure beneath valles caldera new mexico results from the jemez teleseismic tomography experiment new results are presented from the teleseismic component of the jemez tomography experiment conducted across valles caldera in northern new mexico we invert 4872 relative ital p wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km the three principle features of our model for valles caldera are 1 near surface low velocities of minus 17 percent beneath the toledo embayment and the valle grande 2 midcrustal low velocities of minus 23 percent in an ellipsoidal volume underneath the northwest quadrant of the caldera and 3 a broad zone of low velocities minus 15 percent in the lower crust or upper mantle crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast near surface low velocities are interpreted as thick deposits of bandelier tuff and postcaldera volcaniclastic rocks lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast beneath the valle grande we interpret the midcrustal low velocity zone to contain a minimum melt fraction of 10 percent while we cannot rule out the possibility that this zone is the remnant 1 2 ma bandelier magma chamber the eruption history and geochemistry of the volcanic rocks erupted in valles calderamore following the bandelier tuff make it more likely that magma results from a new pulse of intrusion indicating that melt flux into the upper crust beneath valles caldera continues the low velocity zone near the crust mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle in either case this low velocity anomaly indicates that underplating by mantle derived melts has occurred copyright 1998 american geophysical union less

continental scale temperature variability during the past two millennia past global climate changes had strong regional expression to elucidate their spatio temporal pattern we reconstructed past temperatures for seven continental scale regions during the past one to two millennia the most coherent feature in nearly all of the regional temperature reconstructions is a long term cooling trend which ended late in the nineteenth century at multi decadal to centennial scales temperature variability shows distinctly different regional patterns with more similarity within each hemisphere than between them there were no globally synchronous multi decadal warm or cold intervals that define a worldwide medieval warm period or little ice age but all reconstructions show generally cold conditions between ad 1580 and 1880 punctuated in some regions by warm decades during the eighteenth century the transition to these colder conditions occurred earlier in the arctic europe and asia than in north america or the southern hemisphere regions recent warming reversed the long term cooling during the period ad 1971 2000 the area weighted average reconstructed temperature was higher than any other time in nearly 1 400 years

solar signature in sedimentary cycles from the late precambrian elatina formation australia climatic cyclicity is recorded by regular variations in the thickness of varves in the elatina formation a late precambrian periglacial lake deposit in south australia earlier conclusions that the climatic cycles reflect solar variability are strongly supported by a complexity of periods revealed through study of drill cores of the varved sequence the new data promise to contribute significantly to our understanding of solar processes

reliability of lunar orbital periods extracted from ancient cyclic tidal rhythmites abstract ancient tidal periods extracted from the geologic record are useful to constrain and calibrate theoretical models of the evolution of the earth moon system analysis of rocks that contain such tidal periodicities termed cyclic rhythmites have tended to concentrate on long series and have focused on the more obvious periods for example neap spring periods have been the most widely discussed such information can be reliably extracted from bedform thickness series measured within laterally accreted tidal bundles and vertically accreted tidal laminae other lunar orbital data is also encoded in cyclic rhythmites however this additional information has been much less extensively applied to earth moon models herein short segments of well formed cyclic rhythmites are examined to determine how reliably the various tidal periods can be extracted via harmonic analysis rhythmite series analyzed here exhibit well formed neap spring tidal periods and also exhibit a prominent fortnightly inequality or a variation in the height of successive spring tides this inequality is related to the elliptical nature of the lunar orbit anomalistic month and it is demonstrated that this period can also be reliably extracted from cyclic rhythmites tropical periods which reflect the changing lunar declination that occurs during a tropical month have also been reported from cyclic rhythmites unlike synodic and anomalistic periods the apparent tropical periods extracted from some cyclic rhythmites do not always accurately correspond to the expected values in only a few cases can this period be reliably extracted from short segments of cyclic tidal rhythmite data this period is commonly only poorly encoded in rhythmites because 1 it tends to be masked by higher amplitude synodic periods and 2 it is not well expressed within predominantly semidiurnal tidal systems samples selected from the proterozoic and phanerozoic tend to be more common during specific intervals of geologic time these intervals were characterized by glacio eustatic flux and in some cases by unique continental configurations understanding these basic controls on rhythmite occurrences should aid in the discovery of additional examples

abrupt onset of the little ice age triggered by volcanism and sustained by sea ice ocean feedbacks 1 northern hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes however the causes of superposed century scale cold summer anomalies of which the little ice age lia is the most extreme remain debated largely because the natural forcings are either weak or in the case of volcanism short lived here we present precisely dated records of ice cap growth from arctic canada and iceland showing that lia summer cold and ice growth began abruptly between 1275 and 1300 ad followed by a substantial intensification 1430 1455 ad intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium a transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times and that cold summers can be maintained by sea ice ocean feedbacks long after volcanic aerosols are removed our results suggest that the onset of the lia can be linked to an unusual 50 year long episode with four large sulfur rich explosive eruptions each with global sulfate loading 60 tg the persistence of cold summers is best explained by consequent sea ice ocean feedbacks during a hemispheric summer insolation minimum large changes in solar irradiance are not required

a 950 yr temperature reconstruction from duckhole lake southern tasmania australia a lack of quantitative high resolution paleoclimate data from the southern hemisphere limits the ability to examine current trends within the context of long term natural climate variability this study presents a temperature reconstruction for southern tasmania based on analyses of a sediment core from duckhole lake 43 365 s 146 875 e the relationship between non destructive whole core scanning reflectance spectroscopy measurements in the visible spectrum 380 730 nm and the instrumental temperature record ad 1911 2000 was used to develop a calibration in time reflectance spectroscopy based temperature model results showed that a trough in reflectance from 650 to 700 nm which represents chlorophyll and its derivatives was significantly correlated to annual mean temperature a calibration model was developed r 0 56 pauto 0 05 root mean squared error of prediction rmsep 0 21 c five year filtered data calibration period 1911 2000 and applied down core to reconstruct annual mean temperatures in southern tasmania over the last c 950 years this indicated that temperatures were initially cool c ad 1050 but steadily increased until the late ad 1100s after a brief cool period in the ad 1200s temperatures again increased temperatures steadily decreased during the ad 1600s and remained relatively stable until the start of the 20th century when they rapidly decreased before increasing from ad 1960s onwards comparisons with high resolution temperature records from western tasmania new zealand and south america revealed some similarities but also highlighted differences in temperature variability across the mid latitudes of the southern hemisphere these are likely due to a combination of factors including the spatial variability in climate between and within regions and differences between records that document seasonal i e warm season late summer versus annual temperature variability this highlights the need for further records from the mid latitudes of the southern hemisphere in order to constrain past natural spatial and seasonal annual temperature variability in the region and to accurately identify and attribute changes to natural variability and or anthropogenic activities

one thousand seven hundred years of interaction between glacial activity and flood frequency in proglacial lake muzelle western french alps local glacial fluctuations and flood occurrences were investigated in the sediment sequence of proglacial lake muzelle based on geochemical analysis and organic matter content established using loss on ignition and reflectance spectroscopy we identified six periods of increased glacial activity over the last 1700 yr each is in accordance with records from reference glaciers in the alps a total of 255 graded layers were identified and interpreted as flood deposits most of these occurred during glacial advances such as the little ice age period and exhibit thicker deposits characterized by an increase in the fine grain size fraction fine sediment produced by glacial activity is transported to the proglacial lake during heavy rainfall events the excess of glacial flour during these periods seems to increase the watershed s tendency to produce flood deposits in the lake sediment suggesting a strong influence of the glacier on flood reconstruction records thus both flood frequency and intensity which is estimated based on layer thickness as a proxy cannot be used in reconstruction of past extreme events because of their variability there is a need to take into account changes in sediment supply in proglacial areas that could preclude satisfactory interpretation of floods in terms of past climate variability

solar periodicities expressed in varves from glacial skilak lake southern alaska a sequence of 236 varve thickness measurements by perkins and sims from proglacial skilak lake in southern alaska is shown to contain spectral features similar to those of the sunspot index in general the varve spectrum qualitatively mimics that of the sunspot index with discrepancies perhaps arising from the misinterpretation of random summer layers as annual increments varves these discrepancies may be largely removed through maximization of the sunspot index varve cross correlation whereby a time scale factor increment is applied to the varve series measurements varve thickness is positively correlated with the sunspot index and with vanishing lag insofar as can be resolved as varve thickness and meteorological variables have previously been shown to be strongly correlated a direct link between the sunspot index and varve thickness is suggested by way of climate response to solar variations

late precambrian tidal rhythmites in south australia and the history of the earth s rotation sedimentary rhythmites of siltstone and fine sandstone from late precambrian c 650 800 ma glaciogenic formations in south australia are interpreted as distal ebb tidal deposits that record variability in the velocity and range of palaeo ebb tides variations in lamina thickness encode a full spectrum of palaeotidal cycles including semidiurnal diurnal fortnightly and monthly tidal cycles as well as the lunar apsides perigee and nodal cycles a half yearly oscillation is attributable largely to a beat between the fortnightly tidal cycles of luni solar conjunction and lunar declination the lunar nodal cycle is discernible as an amplitude modulation of this beat oscillation the data allow determination of the earth9s palaeorotation and the past dynamics of the earth moon system with an accuracy previously unattainable for the precambrian the late precambrian c 650 ma year contained 13 1 0 5 lunar months and c 400 20 days and the late precambrian lunar month c 30 5 1 5 days these values suggest an average equivalent phase lag near 3 since late precambrian time rather than the present value of 6 the period of 19 5 0 5 years determined for the lunar nodal cycle c 650 ma ago indicates a lunar distance 96 9 1 7 of the present distance the low rate of lunar recession since late precambrian time revealed by the rhythmite data militates against a close approach of the moon during the proterozoic precambrian sedimentary rhythmites may hold a key to the early history of the earth9s rotation

in situ reflectance spectroscopy analysing techniques for high resolution pigment logging in sediment cores the temporal resolution of marine proxy data is limited by analytically required sample size we present in situ reflectance spectroscopy techniques usually applied in remote sensing to analyse the organic fraction of marine and terrestrial sediment from absorption band depths photosynthesis pigment variations are derived for sediments from the upwelling region off peru where productivity is related to the annual variability of el nino strength quantitative estimations of diagenetic photosynthesis pigments derived from absorption band analysis in reflectance spectra are highly correlated to organic carbon content the ratio of pigment fractions is related to chlorine concentration and reflects organic matter preservation and deep water ventilation changes the import of terrigenous mineroclastics tm by local rivers is semi quantitatively documented in the spectrum continuum tm is inversely varying with organic matter preservation and chlorine concentration

a quantitative high resolution summer temperature reconstruction based on sedimentary pigments from laguna aculeo central chile back to ad 850 we present a pigment based quantitative high resolution five years austral summer djf december to february temperature reconstruction for central chile back to ad 850 we used non destructive in situ multichannel reflection spectrometry data from a short sediment core of laguna aculeo 33 50 s 70 54 w 355 m a s l central chile calibration in time period ad 1901 2000 cross validated with split periods revealed robust correlations between local djf temperatures and total sedimentary chlorin relative absorption band depth rabd centred in 660 670 nm rabd 660 670 r 0 79 p 0 01 five years triangular filtered and the degree of pigment diagenesis r660nm 670 nm r 0 82 p 0 01 five years triangular filter root mean squared error values are small between 0 24 and 0 34 c suggesting that most of the reconstructed decadal scale cli mate variability is significant our data provide quantitative evidence for the presence of a medieval climate anomaly in this case warm summers between ad 1150 and 1350 t 0 27 to 0 37 c with respect to wrt twentieth century and a very cool period synchronous to the little ice age starting with a sharp drop between ad 1350 and ad 1400 0 3 c 10 yr decadal trend followed by constantly cool t 0 70 to 0 90 c wrt twentieth century summers until ad 1750 the structure of variability is consistent in great detail with annu ally resolved tree ring based warm season temperature and river discharge reconstructions from northern patagonia 42 s for the past 400 years with qualitative climate reconstructions from andean glacier fluctua tions and with hydrological changes in patagonian lake sediment records