linking the 8 2 ka event and its freshwater forcing in the labrador sea 1 the 8 2 ka event was the last deglacial abrupt climate event a reduction in the atlantic meridional overturning circulation amoc attributed to the drainage of glacial lake agassiz may have caused the event but the freshwater signature of lake agassiz discharge has yet to be identified in 18o of foraminiferal calcite records from the labrador sea calling into question the connection between freshwater discharge to the north atlantic and amoc strength using mg ca paleothermometry we demonstrate that 3 c of near surface ocean cooling masked an 1 0 decrease in western labrador sea 18o of seawater concurrent with lake agassiz drainage comparison with north atlantic 18o of seawater records shows that the freshwater discharge was transported to regions of deep water formation where it could perturb amoc and force the 8 2 ka event

ice sheet sources of sea level rise and freshwater discharge during the last deglaciation we review and synthesize the geologic record that constrains the sources of sea level rise and freshwater discharge to the global oceans associated with retreat of ice sheets during the last deglaciation the last glacial maximum 26 19 ka was terminated by a rapid 5 10 m sea level rise at 19 0 19 5 ka sourced largely from northern hemisphere ice sheet retreat in response to high northern latitude insolation forcing sea level rise of 8 20 m from 19 to 14 5 ka can be attributed to continued retreat of the laurentide and eurasian ice sheets with an additional freshwater forcing of uncertain amount delivered by heinrich event 1 the source of the abrupt acceleration in sea level rise at 14 6 ka meltwater pulse 1a 14 15 m includes contributions of 6 5 10 m from northern hemisphere ice sheets of which 2 7 m represents an excess contribution above that derived from ongoing ice sheet retreat widespread retreat of antarctic ice sheets began at 14 0 15 0 ka which together with geophysical modeling of far field sea level records suggests an antarctic contribution to this meltwater pulse as well the cause of the subsequent younger dryas cold event can be attributed to eastward freshwater runoff from the lake agassiz basin to the st lawrence estuary that agrees with existing lake agassiz outlet radiocarbon dates much of the early holocene sea level rise can be explained by laurentide and scandinavian ice sheet retreat with collapse of laurentide ice over hudson bay and drainage of lake agassiz basin runoff at 8 4 8 2 ka to the labrador sea causing the 8 2 ka event

error analysis of paleosalinity calculations a detailed error analysis is performed for the calculation of open ocean salinity changes from carbonate oxygen 18 proxy data measurement errors are generally negligible however errors due to the goodness of fit of various regressions used in the calculation are significant depending on the method used and on the proxies available error bars 1 for the salinity range from 0 6 to 1 8 in particular expected errors for the midlatitude and tropical salinity differences at the last glacial maximum are around 1 1 to 2 5 the largest errors occur because of the differences between temporal and spatial seawater oxygen 18 salinity relationships and through the use of nonconcurrent temperature measurements methods of improving paleosalinity estimates should focus on eliminating these large sources of uncertainty

investigating the 8 2 ka event in northwestern madagascar insight from data model comparisons abstract the 8 2 ka event is a well known cooling event in the northern hemisphere but is poorly understood in madagascar here we compare paleoclimate data and outputs from paleoclimate simulations to better understand it records from madagascar suggest two distinct sub events 8 3 ka and 8 2 ka that seem to correlate with records from northern high latitude this could indicate causal relationships via changes in the atlantic meridional overturning circulation amoc with changes in moisture source s 18o and changes in the mean position of the inter tropical convergence zone itcz as climate modelling suggests these two sub events are also apparent in other terrestrial records but the climatic signals are different the prominent 8 2 ka sub event records a clear antiphase relationship between the northern and southern hemisphere monsoons whereas such relationship is less evident during the first 8 3 ka sub event data model comparison have also shown a mismatch between the paleoclimate data and the model outputs the causes of which are more or less understood and may lie in the proxies in the model or in both data and model knowing that paleoclimate proxies and climate models produce different sets of variables further research is needed to improve the data model comparison approach so that both paleoclimate data and paleoclimate models will better predict the likely climate status of a region during a specified time in the past with minimal uncertainties

routing of western canadian plains runoff during the 8 2 ka cold event 1 the collapse of the laurentide ice sheet over hudson bay 8 47 ka allowed the rapid drainage of glacial lake agassiz into the labrador sea an event identified as causing a reduction in atlantic meridional overturning circulation amoc and the 8 2 ka cold event atmosphere ocean models simulations based on this forcing however fail to reproduce several characteristics of this event particularly its duration here we use planktonic foraminifera u ca records to document the routing of western canadian plains runoff that accompanied ice sheet collapse geochemical modeling of the 7 nmol mol increase in u ca at the opening of hudson bay indicates an increase in freshwater discharge of 0 13 0 03 sverdrups 106 m3 s 1 from routing a sufficient magnitude to cause an amoc reduction we suggest that this routing event suppressed amoc strength for several centuries after the drainage of lake agassiz explaining multi centennial climate anomalies associated with the 8 2 ka cold event

lake agassiz outburst age and routing by labrador current and the 8 2 cal ka cold event abstract catastrophic drainage of glacial lake agassiz impounded by the laurentide ice sheet lis into hudson bay and strait is widely thought to have caused the 8 2 cal ka cold event though the path of its drainage in the north atlantic is poorly known analysis and compilation of 10 sediment cores from 8 sites distributed on the eastern canadian continental shelf and upper slope south of hudson strait revealed distinct beds with elevated detrital carbonate dc concentrations known by their distinct composition to originate from erosion of carbonate bedrock and carbonate rich glacial sediments in the hudson bay hudson strait region at all but one site radiocarbon ages bracketing and within the thickest and most widespread of these beds are coeval with the age constraints for the agassiz floods in hudson strait and southeastern hudson bay the dc distribution and concentration of this bed are consistent with southward transport in the labrador current transfer function analysis of dinoflagellate assemblage data from cores on the central labrador shelf in hudson bay and on the northeast newfoundland shelf reveal that early holocene seasonal ice cover was 5 6 months longer than at present requiring the application of a 200 year reduction to the radiocarbon age constraints on the lake agassiz outburst floods the age of the floods is reduced to about 8 33 cal ka 7 5 14 c ka from the previous determination of 8 47 cal ka 7 7 14 c ka with a similar reduction in their error ranges to 8 04 8 49 cal ka from 8 16 8 74 cal ka based on present current routing the agassiz floodwaters and icebergs are inferred to have travelled south in the labrador current and to have possibly become incorporated into the north atlantic current and transported to the nordic seas there the freshwater additions would have lessened deepwater production with a concomitant reduction in northward heat transport in the upper limb of the atlantic meridional overturning circulation thereby contributing to the 8 2 cal ka cold event variation in the calcite and dolomite content of the agassiz dc bed two salinity reductions during agassiz bed deposition and low level shorelines of lakes agassiz and ojibway suggest that more than one outburst was delivered during the final agassiz drainage the 8 2 cal ka cold event registered in greenland was coeval with a second or later flood which possibly accompanied the final collapse of the lis in hudson bay

insights on the events surrounding the final drainage of lake ojibway based on james bay stratigraphic sequences abstract deglaciation of the james bay region was highly dynamic with the occurrence of ice cochrane readvances into glacial lake ojibway around final deglaciation time which culminated with the drainage of ojibway waters into hudson bay and subsequent incursion of the tyrrell sea at 8 ka renewed interest on these events comes from the possible link between the drainage of the ice dammed lake agassiz ojibway and a major climate deterioration known as the 8 2 ka cooling event recent glaciological modeling suggests that this drainage may have occurred subglacially a mechanism that can accommodate more than one lake discharge as suggested by marine records the exact number and timing of drainage events as well as location of the lake discharge pathway s remain however largely unconstrained here we focus on the events that led to the drainage of lake ojibway by documenting late glacial sedimentary sequences located east of james bay our investigations indicate that the deglacial sequence consists of a readvance till extensive ojibway rhythmites and thick marine sediments the glaciolacustrine and marine units are separated by a 60 cm thick horizon composed of laminated silt beds containing rounded clay balls and disseminated clasts resulting from the abrupt drainage of the lake radiocarbon dating of marine fossils lying above the drainage horizon indicates that the glaciolacustrine episode ended around 8128 8282 cal yr bp micropaleontological analyses reveal that freshwater ostracods candona sp and marine microfossils foraminifers dinocysts occur together in the upper part of the ojibway sediments analysis of oxygen isotopes 18 o of ostracods and foraminifers originating from the same stratigraphic position show highly contrasting values that suggest possible subglacial exchanges between lake ojibway and tyrrell sea waters prior to the final drainage event the complexity of the deglacial events is further indicated by radiocarbon dating of marine shells retrieved from a cochrane till that suggests that the last ice readvance occurred almost simultaneously with the final lake discharge these results bring additional constraints on the drainage mechanism of the coalesced lake agassiz ojibway and indicate that the james bay region formed an important drainage pathway for meltwaters at the end of the last deglaciation

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

large sensitivity to freshwater forcing location in 8 2 ka simulations the 8 2 ka event is a key test case for simulating the coupled climate response to changes in the atlantic meridional overturning circulation amoc recent advances in quantifying freshwater fluxes at 8 2 ka from the proxy record have improved the realism of the forcing magnitude in model simulations yet this forcing is still generally applied in an unrealistic geographic manner across most of the labrador sea rather than just along the labrador coast previous simulations with eddy resolving ocean models have come to conflicting conclusions regarding the ability of such a coastally confined flow to impact the amoc these simulations have also not incorporated full atmosphere models nor have they used the new meltwater forcing values for 8 2 ka we use the community climate system model version 3 with an ocean model resolution only slightly coarser than that used in previous eddy resolving simulations to test the sensitivity to freshwater forcing location when revised freshwater forcing is applied across the labrador sea the amoc is reduced by 40 and climate anomalies compare well with proxy records of the 8 2 ka event in terms of magnitude and duration when the forcing is added just along the labrador coast however most meltwater joins the subtropical gyre and travels to the subtropics with minor impact to the amoc 10 decrease it is likely that model biases in the placement of the north atlantic current remain an important limitation for correctly simulating the 8 2 ka event

plant wax δd values record changing eastern mediterranean atmospheric circulation patterns during the 8 2 kyr b p climatic event abstract throughout the holocene the climate of the mediterranean region has been strongly influenced by variability in the atmospheric circulation of the high and low latitudes a prominent example for such holocene climate perturbations is the 8 2 kyr b p climatic event reorganization of northern hemisphere atmospheric circulation patterns resulted in variations of temperature and precipitation distribution across the mediterranean the effects of changing high and low latitude atmospheric circulation on mediterranean climate in relation to the 8 2 kyr b p climatic event are however not well understood here we present a high resolution record of stable hydrogen isotope composition of plant wax n alkanes dwax across the 8 2 kyr b p climatic event from the tenaghi philippon peat deposit ne greece in order to characterize patterns of precipitation and changes in atmospheric circulation in the eastern mediterranean region our record reveals pronounced changes in dwax that correlate closely with previously published palynological data a long term decline in dwax values characterizes the lower part of the section the 8 2 kyr b p climatic event itself is connected to two distinct positive dwax excursions a minor shift in dwax around 8 2 kyr b p and a major shift in dwax between ca 8 1 and 8 0 kyr b p the upper part of the section shows a progressive trend towards higher dwax values we link shifts in dwax to changes in mediterranean air mass trajectories supplying precipitation to ne greece caused by variations in the relative contributions of northerly derived d depleted moisture and southerly derived d enriched moisture possible control mechanisms for alternating air mass trajectories include changes in the influence of the siberian high and differences in the influence of the african and asian monsoon circulation on anticyclonic conditions in the mediterranean region as well as regional inflow of moist air masses from the aegean sea

oxygen isotope precipitation anomaly in the north atlantic region during the 8 2 ka event an annual laminated stalagmite from northern spain recorded two outstanding oxygen isotope 18o pulses during the 8 2 ka event most of the oxygen isotope record variability is related to the amount of rainfall although other factors affect the signal the lamina thickness record which is also related to amount of rainfall does not show any significant anomaly at the time of oxygen isotope spikes considering the factors affecting the isotope signal the two prominent falls in 18o during the event are interpreted to be caused by the release of large amounts of fresh waters into the north atlantic thus 18o depleted ocean surface waters shifted the rainfall 18o composition for europe and greenland our precise chronology provides the timing of the outbursts that caused the 18o anomalies at 8350 8340 and 8221 8211 34 yr b p most probably generated by the drainage of proglacial lakes agassiz and ojibway therefore in the north atlantic region 18o records during the 8 2 ka event trace important hydrological modifications in the ocean not just local climate as a consequence paleoclimate reconstructions from europe and greenland using the 18o proxy that does not take this into account would be overestimating the magnitude of the anomalies during the 8 2 ka event

delta15n measurements as a calibration tool for the paleothermometer and gas ice age differences a case study for the 8200 b p event on grip ice isotopic composition measurement on atmospheric nitrogen 15n trapped in ice cores is established as a gravitational enrichment indicator recently it was discovered that thermal diffusion has a significant influence on this parameter during periods of large temperature fluctuations severinghaus et al 1998 here we present results for the 8200 b p event the largest temperature excursion during the holocene which underpin this view and extend it to medium range temperature changes the thermal diffusion component is about one third of the gravitational enrichment for this event the shortness of this cooling event does not significantly affect the gravitational enrichment as documented by rather constant close off depths therefore the 15n residuals can be directly used to estimate temperature changes hence calibrating the paleothermometer since our 15n record can resolve temperature variations as short as a few decades it complements the calibration technique of borehole temperature measurements for which the resolution decreases rapidly with increasing age we obtained a relation 18oice temperature of 0 30 0 11 k which agrees nicely with the temporal dependence for cold periods about 0 30 k and is significantly lower than today s greenland spatial relation 0 67 k this corresponds to a range of the surface temperature shift for the 8200 b p event in central greenland between 5 4 and 11 7 k with a best estimate of 7 4 k by matching the 15n variations with 18oice variations we estimate the mean gas ice age difference to be 209 15 years which is in good agreement with model estimates

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

lake isotope records of the 8200 year cooling event in western ireland comparison with model simulations the early holocene cooling which occurred around 8200 calendar years before present was a prominent abrupt event around the north atlantic region here we investigate the timing duration magnitude and regional coherence of the event as expressed in carbonate oxygen isotope records from three lakes on northwest europe s atlantic margin in western ireland namely loch avolla loch gealain and lough corrib an abrupt negative oxygen isotope excursion lasted about 200 years comparison of records from three sites suggests that the excursion was primarily the result of a reduction of the oxygen isotope values of precipitation which was likely caused by lowered air temperatures possibly coupled with a change in atmospheric circulation comparison of records from two of the lakes loch avolla and loch gealain which have differing bathymetries further suggests a reduction in evaporative loss of lake water during the cooling episode comparison of climate model experiments with lake sediment isotope data indicates that effective moisture may have increased along this part of the northeast atlantic seaboard during the 8200 year climatic event as lower evaporation compensated for reduced precipitation

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

global calibration of ecological models for planktic foraminifera from coretop carbonate oxygen 18 abstract a global dataset of coretop planktic foraminiferal 18oc is combined with a global database of seawater 18ow observations and ocean climatologies to determine robust optimum parameters for an ecological model for six commonly measured species and varieties these parameters consist of the temperature ranges optimum temperatures depth habitat and the amount of secondary calcification for globigerinoides ruber white g ruber pink neogloboquadrina pachyderma l n pachyderma r globigerinoides sacculifer and globigerina bulloides this approach produces ecological models and temperature ranges consistent with previous work and manages to reproduce the coretop oxygen 18 carbonate values remarkably well the standard error of modelled values of coretop calcite globally and for all species is 0 53 compared to an error of 1 2 when assuming annual average mixed layer equilibrium calcite

sensitivity of sea ice to physical parameterizations in the giss global climate model 1 the giss coupled model is used to investigate the sensitivity of sea ice to each of the following parameterizations 1 two sea ice dynamics cf cavitating fluid vp viscous plastic 2 the specification of oceanic isopycnal mixing coefficients in the gent and mcwillams isopyncal mixing gm and 3 the wajsowicz viscosity diffusion wv the large scale sea ice properties are highly sensitive to sea ice dynamics with the inclusion of resistance to shear stress vp captures the major observed sea ice drift features and improves the simulations of sea ice concentrations thickness and export through fram strait relative to cf gm significantly improves the simulation of vertical temperature distributions in the southern ocean although it leads to a dramatic reduction of antarctic sea ice cover the reduced oceanic isopycnal mixing coefficients lead to arctic sea ice that tends to be less and thinner in almost the entire arctic except in the north pacific and labrador sea while antarctic sea ice that extends more equatorward throughout the circumpolar regions the responses of sea ice to wv show an enlargement and thickening of sea ice in the arctic within the ice packs around the antarctic and a reduction and thinning of sea ice in the northern weddell and ross seas on the basis of these experiments two composite experiments with the best parameterizations are investigated the atmospheric responses associated with sea ice changes are discussed while improvements are seen there are still many unrealistic aspects that will require further improvements to sea ice and ocean components

simulation of the cold climate event 8200 years ago by meltwater outburst from lake agassiz meltwater pulse is assumed to have a volume of 1 6 10 14 m 3 and a period of discharge of 2 years on the basis of glaciological modeling of the decay of the laurentide ice sheet lis we present a possible mechanism which can explain the centennial duration of the 8 2 ka cold event the mechanism is related to the existence of an additional equilibrium climate state with reduced north atlantic deep water nadw formation and a southward shift of the nadw formation area hints at the additional climate state were obtained from the largely varying duration of the pulse induced cold episode in response to overlaid random freshwater fluctuations in monte carlo simulations the model equilibrium state was attained by releasing a weak multicentury freshwater flux through the st lawrence pathway completed by the meltwater pulse the existence of such a climate mode appears essential for reproducing climate anomalies in close agreement with paleoclimatic reconstructions of the 8 2 ka event the results furthermore suggest that the temporal evolution of the cold event was partly a matter of chance index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4255 oceanography general numerical modeling 4532 oceanography physical general circulation 4215 oceanography general climate and interannual variability 3309 9325 information related to geographic region atlantic ocean

evidence for solar influence in a holocene speleothem record pere noel cave se belgium abstract we present a decadal centennial scale holocene climate record based on trace elements contents from a 65 cm stalagmite from belgian pere noel cave pere noel pn stalagmite covers the last 12 7 ka according to u th dating high spatial resolution measurements of trace elements sr ba and mg were done by laser ablation inductively coupled plasma mass spectrometry la icp ms trace elements profiles were interpreted as environmental and climate changes in the han sur lesse region power spectrum estimators and continuous wavelet transform were applied on trace elements time series to detect any statistically significant periodicities in the pn stalagmite spectral analyses reveal decadal to millennial periodicities i e 68 75 133 136 198 209 291 358 404 602 912 1029 and 2365 2670 yr in the speleothem record results were compared to reconstructed sunspot number data to determine whether solar signal is presents in pn speleothem the occurrence of significant solar periodicities i e cycles of gleissberg de vries unnamed 500 years eddy and hallstatt supports for an impact of solar forcing on pn speleothem trace element contents moreover several intervals of significant rapid winter change were detected during the holocene at 10 3 9 3 9 5 around 8 2 6 4 6 2 4 7 4 5 and around 2 7 ka bp those intervals are similar to the cold winter events evidenced in different natural paleoclimate archives suggesting common climate forcing mechanisms related to changes in solar irradiance

cooling and freshening at 8 2 ka on the nw iceland shelf recorded in paired δ18o and mg ca measurements of the benthic foraminifer cibicides lobatulus a shallow marine sediment core from nw iceland provides evidence for a brief cooling and freshening at 8200 cal yr bp consistent with the hypothesis that the catastrophic outburst flood of the proglacial lakes oijbway and agassiz caused the 8 2 ka event this is the first high resolution record reconstructing near surface temperatures and 18osw by paired measurements of mg ca and 18ocalcite of a benthic foraminifer we developed a new mg ca temperature calibration for cibicides lobatulus our down core mg ca derived temperature reconstruction dates the 8 2 ka cooling event between 8300 cal yr bp and 8100 cal yr bp which is similar to the timing and 160 yr duration recorded in the greenland ice cores the near surface temperature drop of 3 to 5 c during the 8 2 ka event was accompanied by lighter 18osw values synchronously to the changes in the geochemical proxies the percentages of two arctic benthic foraminifers increased and the percent calcium carbonate decreased our record combined with several others from the region suggests that the freshwater outburst spread far from the source into the high latitude north atlantic this freshwater input could have directly caused substantial high latitude cooling with reduced north atlantic deep water formation amplifying the climatic impact

a regional stratigraphic isochron ca 800014c yr b p from final deglaciation of hudson strait sedimentologic rock magnetic and x ray fluorescence data from two marine sediment cores in hudson strait suggest that a red hematite rich clay layer was deposited throughout the strait during the final collapse of the laurentide ice sheet in the vicinity of northern hudson bay and western hudson strait this layer which can be recognized by its reddish pink color 10yr6 2 to 5yr4 2 and relatively high hematite proportions low magnetic susceptibility and magnetite to hematite ratio is dated from 8000 to 790014c yr b p at both ends of the strait the dubawnt group a proterozoic bedrock unit in northern hudson bay is the most likely source of this stratigraphic isochron in eastern hudson strait the recognition of this red unit and other distal glaciomarine sediments from 8400 to 790014c yr b p indicates that little sediment from the nearby labrador dome reached eastern hudson strait during this 500 yr interval this time interval immediately postdates the noble inlet advance a northward flow of labrador ice across eastern hudson strait onto southern baffin island from ca 8900 to 840014c yr b p one explanation for the lack of labrador sediments is that the northern margin of the labrador dome was cold based for up to 500 yr following the noble inlet advance

magnitude of isotope temperature scaling for interpretation of central antarctic ice cores 1 the conventional interpretation of ice core deuterium and oxygen 18 isotope profiles based on the use of present day observations spatial slope underestimates glacialinterglacial surface temperature changes in central greenland by up to a factor of two this likely results from changes in the seasonality of the precipitation due to the particular location of the greenland ice sheet next to the highly variable northern polar front in this regard the situation is much simpler for central antarctica and this should be reflected in the temperature interpretation of ice core isotopic records with this in mind we closely examine all relevant information focusing on the east antarctic plateau where both model and empirical isotope temperature estimates are available we point to the fact that correctly accounting for the influence of ocean isotopic change is important when interpreting deuterium profiles from ice cores in this region the evidence presently available indicates that unlike for greenland the present day spatial slope can probably be taken as a surrogate of the temporal slope to interpret glacial interglacial isotopic changes at sites such as vostok and epica dome c corresponding temperature changes are within 10 to 30 of those obtained from the conventional interpretation based on the use of the spatial slope index terms 1040 geochemistry isotopic composition chemistry 1827 hydrology glaciology 1863 3344 meteorology and atmospheric dynamics paleoclimatology 9310 information related to geographic region antarctica keywords water isotopes temperature estimate antarctica deuterium oxygen 18

the structure of holocene climate change in mid latitude north america abstract a sequence of long term and rapid changes during the holocene appears in a network of 40 well resolved paleoclimate datasets from mid latitude north america including records of pollen inferred temperatures alkenone derived sea surface temperatures ssts lake level changes dust accumulation and lake isotopes from idaho to maine statistical analyses reveal that changes in insolation and the laurentide ice sheet explain 51 7 of the variance in the records especially multi millennial trends but peak rates of change indicate additional rapid changes at ca 10 8 9 4 8 3 7 0 5 5 5 2 4 7 2 1 and 0 9 ka step changes between 9 4 and 8 3 ka relate to ice sheet dynamics that warmed much of the region and changes at 5 5 ka were the largest since the demise of the ice sheet the shift at 5 5 ka initiated widespread cooling and increases in effective moisture which culminated in the coolest wettest millennia in most areas after 2 1 ka replicated evidence from multiple records also shows a spatially varied set of multi century fluctuations including 1 low temperatures and high effective moisture at 5 5 4 8 ka in the mid continent and 2 repeated phases of low ssts cool summers and drought superimposed upon long cooling moistening trends in eastern north american since 5 5 ka

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

distribution patterns of contaminants in the mogale gold tailing dam a case study from south africa this study evaluates the geochemical and mineralogical characterisation of weathering layers associated with the mogale tailing dam in the randfontein cluster witwatersrand basin south africa in the tailing dam it is observed that high hematite fe2o3 contents separate the ferruginous from the rest layers also the oxidised layers with high quartz sio2 contents compositionally differ from the underlying capillary zone grey and ferruginous layers likewise a combination of gypsum contents mno cao tot s versus pyrophyllite muscovite al2o3 and k2o distinguishes the lower grey from the upper grey layer which corresponds to the saturated and capillary zones respectively similarly the amount of ree and most trace elements depleted in the oxidised layers is compared to the underlying layers factor analysis of this variation in layers related element distribution to the spatial patterns of pyrite gypsum hematite and jarosite furthermore analysis of the mass balance revealed an estimated net loss of between 23 and 123 ppm for uranium zinc nickel and arsenic right from the inception of the tailing dam 50 years ago a loss of between 1 5 1 8 and 2 8 2 9 is also estimated for total sulphur and aluminium respectively these results underpin the sources and patterns of mobilisation of elements in the weathering zones as a major step towards the development of a predictive model for mitigation of amd in the region

modeling the effect of freshwater pulses on the early holocene climate the influence of high frequency climate variability freshwater 4 67 10 14 m 3 into the labrador sea at three different constant rates 1 5 sv 1 sv 1 10 6 m 3 s 1 i n 10 years 0 75 sv in 20 years and 0 3 sv in 50 years for each rate five ensemble experiments have been performed varying in initial conditions the freshwater pulses produce a weakening of the thermohaline circulation the perturbed state is in agreement with proxy evidence for the 8 2 ka event two types of recovery of the thermohaline circulation occurred differing in time scale 1 200 years and 2 200 years in the experiments with 10 year and 20 year pulses both types of recovery were observed this suggests that the model response is unpredictable in the range of parameters studied here it is hypothesized that the unpredictability is associated with annual to decadal climate variability our results demonstrate that several types of recovery may exist with the same kind of perturbation the interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere sea ice ocean system index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4255 oceanography general numerical modeling 4532 oceanography physical general circulation 4215 oceanography general climate and interannual variability 3309 9325 information related to geographic region atlantic ocean keywords holocene thermohaline circulation predictability meltwater coupled climate model

millennial scale changes of surface and deep water flow in the western tropical atlantic linked to northern hemisphere high latitude climate during the holocene there is increasing evidence that the preceding holocene climate was as unstable as the last glacial period although variations occurred at much lower amplitudes however low latitude climate records that confirm this variability are sparse here we present a radiocarbon dated holocene marine record from the tropical western atlantic aragonite dissolution derived from the degree of preservation of the pteropod limacina inflata records changes in the corrosiveness of the bottom water at the core site due to the changing influence of northern versus southern water masses the 18 o difference between the shallow living planktonic foraminifera globigerinoides sacculifer and the deep living globorotalia tumida is used as proxy for changes in the vertical stratification of the surface water hence the trade wind strength at this latitude we compared our data to high latitude records of the north atlantic region a good agreement is found between the aragonite dissolution and the strength in the island scotland overflow water which contributes significantly to the north atlantic deep water this suggests that large scale variations in the atlantic thermohaline circulation occurred throughout the holocene concurrently the comparison of our 18 o with the gisp2 glaciochemical records points to global holocene atmospheric reorganizations seen in both the tropics and high northern latitudes

terrestrial impact of abrupt changes in the north atlantic thermohaline circulation early holocene uk abrupt cooling events are features of holocene climate and may recur in the future we use lake records from hawes water nw england to quantify the impact of two prominent early holocene climatic events subdecadal oxygen isotope records from sedimentary carbonate 18 oc dated using thermal ionization mass spectrometry tims u series analyses provide evidence for abrupt cold events lasting 50 and 150 yr at 9350 and 8380 yr ago which correlate with the 9 3 ka and 8 2 ka events recognized in greenland ice cores at hawes water mean july air temperatures inferred from chirono mid assemblages decreased by 1 6 c during each event calculations show that the isotopic excursions were dominantly caused by decreases in the isotopic composition of meteoric precipitation 18 op by 1 3 this is interpreted as a direct downstream response to cooling and freshening of northeast atlantic surface water by melting ice sheets intermediate in magnitude between events observed in greenland and central europe the effects are consistent with a partial shutdown of the north atlantic thermohaline circulation

paleoenvironmental insights into the quaternary evolution of the southern brazilian coast based on fossil and modern diatom assemblages abstract diatom assemblages provide a strong basis for detailed interpretations of paleoceanography and diatom paleoecology of the southern brazilian coast nine cores obtained in the coastal plain shelf and continental slope and thirteen surface sediment samples of the patos lagoon provide an excellent opportunity to use a paleoecological approach to study detailed quaternary environmental changes in the southern coasts in the interval studied the basin sedimentation occurred in a marine dominated environment related to humid periods prior to 43 500 year bp abundant diatom taxa are highlighted as excellent indicators of environmental changes including salinity sediment composition and transport by incised valleys providing additional insights into coastal evolution paralia sulcata is dominant in all analyzed cores occurring continuously from late pleistocene to holocene but is rare in the modern sediments non marine diatom assemblages occurred continuously but in very low densities in periods before and after the last glacial maximum abrupt changes in sedimentation have been recorded related to shifts from marine dominated to shallow estuarine deposition during the holocene sea level rose rapidly which is responsible for the progressive coastal flooding prior to 8420 7930 year bp the holocene barrier system became more continuous and patos lagoon developed consequently the secondary former inlets from the jacui and camaqua rivers are closed and only one single inlet rio grande channel remains active the spatial distribution of modern assemblages is controlled by salinity gradient wind action and climate forcing the bottom sediments of patos lagoon are dominated by freshwater diatom aulacoseira veraluciae aulacoseira sp 2 and marine estuarine species of cyclotella litoralis

sheath fold formation around slip surfaces terra nova 24 417 421 2012 abstract sheath folds are associated with shear zones and have been used as high strain indicators we present results from a three dimensional analytical study showing that the flow perturbation around a planar weakness acting as a slip surface in simple shear leads to the formation of sheath folds at low shear strain closed traces of marker layers are exhibited in cross sections cut perpendicular to the shear direction in the vicinity of the slip surface tip in cross sections cut parallel to the shear direction and normal to the shear plane flanking folds are observed our model captures the first order observations of natural sheath folds such as diversity of shapes and multiple eye structures natural equivalents of the planar weaknesses that we model include fractures veins and rheologically weak layers flow perturbation around these zones of weaknesses is proposed as an alternative mechanism of sheath fold formation which emphasizes the coeval operation of brittle and ductile processes in shear zones

the 8k event cause and consequences of a major holocene abrupt climate change abstract a prominent abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern hemisphere regions especially in wintertime in response to a very large outburst flood that freshened the north atlantic changes were much larger than typical climate variability before and after the event with anomalies up to many degrees contributing to major displacement of vegetative patterns this 8k event provides a clear case of cause and effect in the paleoclimatic realm and so offers an excellent opportunity for model testing the response to north atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following north atlantic freshening and so greatly strengthens the case that those older events also reflect north atlantic changes the north atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming caused ice melt and hydrologic cycle intensification at high latitudes lead to major changes in north atlantic circulation few model experiments have directly addressed the 8k event and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies so much work remains to be done

estimating the regional climate signal in a late pleistocene and early holocene lake sediment δ18o record from vermont usa abstract we present a new oxygen isotope 18o record from carbonate rich lake sediments from central vermont the record from twin ponds spans from 13 5 cal ka bp 1950 ad to present but contains a 6 ka long hiatus starting shortly after 7 5 cal ka bp we compare the record for ca 13 5 7 5 cal ka bp with published 18o data from the region after using a bayesian approach to produce many possible chronologies for each site principal component analysis then identified chronologically robust multi site oxygen isotope signals including negative values during the younger dryas but no significant deviations from the early holocene mean of the regional records however differences among sites indicate significant trends that likely relate to interacting changes in the regional gradients of seasonal temperatures and precipitation as well as moisture sources moisture pathways and aridity that were controlled by large scale climatic controls such as insolation the progressive decline of the laurentide ice sheet and changes in oceanic circulation centennial shifts punctuate these trends at ca 9 3 and 8 2 cal ka bp and reveal that the local character of these short lived features requires a detailed understanding of lake hydrology and regional isotopic gradients to yield reliable information for regional climate reconstructions

amplitude and timing of temperature and salinity variability in the subpolar north atlantic over the past 10 k y paired planktic foraminiferal 18 o and mg ca data reveal trends of increasing temperatures 3 c and salinities in the subpolar north atlantic over the course of the holocene which were punctuated by abrupt events the trends likely refl ect an insolation forced northward retreat of the boundary between polar and north atlantic subsurface waters the superimposed variability does not appear to be periodic but tends to recur within a broad millennial band the records provide convincing evidence of open ocean cooling nearly 2 c and freshening during the 8 2 ka event and suggest similar conditions at 9 3 ka however the two largest temperature oscillations in our record 2 c occurred during the past 4 k y suggesting a recent increase in temperature variability relative to the mid holocene perhaps in response to neoglaciation which began at about this time

holocene and last interglacial climate of the faroe islands from sedimentary plant wax hydrogen and carbon isotopes abstract the last interglacial period lig is earth s most recent globally warm period and is analogous in some ways to projected future global warming however questions remain regarding the state of the climate during the lig in the north atlantic a region that is extremely sensitive to changes in oceanic and atmospheric circulation here we present hydrogen and carbon isotope d and 13c records from a suite of plant wax biomarkers preserved in holocene and lig lacustrine sediments from the north atlantic faroe islands and interpret them as qualitative proxies for temperature and hydroclimate variability these data are used to directly compare lig and holocene climate using the same proxy approaches from the same terrestrial location measuring multiple isotopes on multiple types of waxes elucidates the sources of homologous plant waxes we deduce that the d values of long chain n alkanes c27 c33 and mid chain n alkanes c23 c25 in these sedimentary archives reflect leaf water and lake water d values respectively while the d values for both long chain c28 c30 and mid chain n alkanoic acids c24 c26 primarily represent lake water d values plant wax inferred d values of precipitation during the early holocene 10 100 to 8 200 cal yr bp are 35 more positive than late holocene values and decline over the holocene d inferred hydrologic change and 13c inferred plant water use efficiency both indicate that the faroe islands became drier throughout the holocene comparison with measurements from lig plant waxes indicates that late lig in the faroe islands was hydrologically similar to the early to mid holocene 8 200 to 4 000 cal yr bp with enriched precipitation isotopes and reduced evapotranspiration indicating a warmer wetter environment

the cold event 8200 years ago documented in oxygen isotope records of precipitation in europe and greenland stable oxygen isotope ratios of ostracod valves in late glacial and holocene sediments of core as 92 5 from deep lake ammersee southern germany reflect variations of mean oxygen isotope ratios in past atmospheric precipitation the record reconfirms the strong similarity of climate evolution in europe and greenland during the last deglaciation for the first time in europe we find a 200 year long negative 18o excursion which is contemporaneous with the strongest negative 18o excursion in the greenland ice around 8 2 ky before present the 8 2 ky isotopic event on both sides of the north atlantic ocean is interpreted as a cold period most probably induced by a perturbation of the north atlantic thermohaline circulation we discuss two possible triggering mechanisms 1 weak forcing as proposed by alley et al and 2 forcing by a strong and sudden freshwater pulse from the collapse of the hudson ice dome

structure and origin of holocene cold events abstract the present interglacial the holocene spans the period of the last 11 700 years it has sustained the growth and development of modern society the millennial scale decreasing solar insolation in the northern hemisphere summer lead to northern hemisphere cooling a southern shift of the intertropical convergence zone itcz and a weakening of the northern hemisphere summer monsoon systems on the multidecadal to multicentury scale periods of more stable and warmer climate were interrupted by several cold relapses at least in the northern hemisphere extra tropical area based on carefully selected 10 000 year long time series of temperature and humidity precipitation as well as reconstructions of glacier advances the spatiotemporal pattern of six cold relapses during the last 10 000 years was analysed and presented in form of a holocene climate atlas hoclat see http www oeschger unibe ch research projects holocene atlas a clear cyclicity was not found and the spatiotemporal variability of temperature and humidity precipitation during the six specific cold events 8200 6300 4700 2700 1550 and 550 years bp was very high different dynamical processes such as meltwater flux into the north atlantic low solar activity explosive volcanic eruptions and fluctuations of the thermohaline circulation likely played a major role in addition internal dynamics in the north atlantic and pacific area including their complex interaction were likely involved

seasonal resolution of eastern mediterranean climate change since 34 ka from a soreq cave speleothem the combination of ion microprobe analysis of d 18 o and confocal laser fluorescent microscope imaging of annual growth bands in a soreq cave speleothem provides sub annual scale climate information between 34 and 4 ka this high resolution methodology is ideal both for comparing seasonal climate patterns across broad windows of time and examining rapid climate events such as the younger dryas termination in detail the sub annual d 18 o gradients we report represent a combination of seasonal variability in rainfall amount and air temperature a distinct change in both the pattern of fluorescent banding and the gradient of d 18 o measured in situ across single annual growth bands indicates a change in seasonal climate patterns of the eastern mediterranean region following heinrich event 1 and again after the younger dryas throughout the holocene wet winters and dry summers characterized regional climate during the younger dryas we find that regional climate may have been more arid than in the holocene but the fluorescent banding pattern indicates that the supply of dripwater to the cave was more consistent year round we suggest that a reduced gradient of seasonal precipitation occasional snowfall and vegetation differences may have all contributed to the isotope and fluorescent banding patterns observed during heinrich event 1 and the last glacial stadial detailed investigation of the younger dryas termination reveals a rapid onset of regional environmental change fluorescent band counting indicates that the younger dryas termination as recorded by rainfall in the

quantification of the geometrical parameters of non cylindrical folds abstract the geometrical parameters of natural folds are used by structural geologists to estimate finite strain and rheological properties of deformed rocks the relation between geometry and rheology is well understood in cases of cylindrical folds but is still limited for non cylindrical folds although the latter are frequent in nature the sparsity of quantitative geometrical data of non cylindrical folds can be explained by the small number of 3d exposures and by the lack of robust methods to quantify their geometrical parameters in 3d space we present a new workflow which can be used to quantify geometrical parameters of non cylindrical folds 3d fold geometry is described using fold wavelength arc length l and amplitude a as most natural folds do not show ideal shapes but are affected by various types of discontinuities the new procedure is not fully automatic but requires the manual selection of measuring profiles along which the geometrical parameters are constrained the new workflow is tested using natural and experimentally produced non cylindrical folds the geometric parameters obtained can be used to improve our understanding of fold kinematics and fold mechanics and should assist the quantitative analysis of non cylindrical folds present in gneiss and salt domes and in rocks containing reservoirs of hydrocarbons and minerals deposits

large arctic temperature change at the wisconsin holocene glacial transition analysis of borehole temperature and greenland ice sheet project ii ice core isotopic composition reveals that the warming from average glacial conditions to the holocene in central greenland was large approximately 15 c this is at least three times the coincident temperature change in the tropics and mid latitudes the coldest periods of the last glacial were probably 21 c colder than at present over the greenland ice sheet

timing and structure of the 8 2 kyr b p event inferred from δ18o records of stalagmites from china oman and brazil oxygen isotope records of stalagmites from china and oman reveal a weak summer monsoon event with a double plunging structure that started 8 21 0 02 kyr b p an identical but antiphased pattern is also evident in two stalagmite records from eastern brazil indicating that the south american summer monsoon was intensifi ed during the 8 2 kyr b p event these records demonstrate that the event was of global extent and synchronous within dating errors of 50 years in comparison with recent model simulations it is plausible that the 8 2 kyr b p event can be tied in changes of the atlantic meridional overturning circulation triggered by a glacial lake draining event this in turn affected north atlantic climate and latitudinal position of the intertropical convergence zone resulting in the observed low latitude monsoonal precipitation patterns

holocene climate variability in antarctica based on 11 ice core isotopic records a comparison is made of the holocene records obtained from water isotope measurements along 11 ice cores from coastal and central sites in east antarctica vostok dome b plateau remote komsomolskaia dome c taylor dome dominion range d47 km105 and law dome and west antarctica byrd with temporal resolution from 20 to 50 yr the long term trends possibly reflect local ice sheet elevation fluctuations superimposed on common climatic fluctuations all the records confirm the widespread antarctic early holocene optimum between 11 500 and 9000 yr in the ross sea sector a secondary optimum is identified between 7000 and 5000 yr whereas all eastern antarctic sites show a late optimum between 6000 and 3000 yr superimposed on the long time trend all the records exhibit 9 aperiodic millennial scale oscillations climatic optima show a reduced pacing between warm events typically 800 yr whereas cooler periods are associated with less frequent warm events pacing 1200 yr

lake agassiz drainage bracketed holocene hudson bay ice saddle collapse abstract the duration and intensity of the early holocene freshening events has been explained by subglacial outburst s from lake agassiz the collapse of a laurentide ice sheet saddle overlying hudson bay and or a combination of the two events our field evidence provides new radiocarbon ages and geomorphologic observations to assess the deglacial history of this important region allowing for revision of the sequence of events we show that the collapse of the hudson bay ice saddle in southwestern hudson bay occurred between 8 57 0 28 ka bp and 8 11 0 19 ka bp this event was preceded by at least one subglacial drainage event through numerous newly mapped subglacial channels onshore of southwestern hudson bay lake agassiz may have experienced multiple subglacial drainage events prior to the final hbis collapse accounting for the timing discrepancies in freshwater cooling observed in the north atlantic importantly this new work links the chronology of events on the southwest land based side of the hbis to the northeast marine based side of the ice sheet additionally this work provides a potential analog for the behaviour of other ice sheets whose beds lie well below sea level such as the west antarctic ice sheet during periods of warming climate

drifting arctic sea ice archives changes in ocean surface conditions 18o profiles in drifting arctic sea ice are coupled with back trajectories of ice drift and an ice growth model to reconstruct the surface hydrography of the arctic ocean interior the results compare well with 18o values obtained by traditional oceanographic methods and known water mass distributions analysis of the stable isotopic composition of sea ice floes sampled at strategic and relatively accessible locations e g fram strait could aid in mapping spatial and temporal variations in arctic ocean surface waters

the 8 2 ka event from greenland ice cores we present a collection of high resolution chemistry and stable isotope records from the plateau of the greenland ice cap during the cold event 8200 yr ago using a composite of four records the cold event is observed as a 160 5 yr period during which decadal mean isotopic values were below average within which there is a central event of 69 yr during which values were consistently more than one standard deviation below the average for the preceding period four cores in north south and central greenland show differences at decadal and shorter timescales it is not yet clear if this represents significant spatial differences in response the results show clear evidence for colder temperatures and a decrease in snow accumulation rate however the changes in chemical concentrations for the ions looked at here are small suggesting only minor changes in atmospheric circulation for this event apart from the decrease in methane concentration greenland ice cores give only weak evidence for effects outside the north atlantic region

the 8 2 ka event abrupt transition of the subpolar gyre toward a modern north atlantic circulation 1 climate model simulations of the 8 2 ka event show an abrupt strengthening of the atlantic subpolar gyre that allows us to connect two major but apparently contradictory climate events of the early holocene the freshwater outburst from proglacial lakes and the onset of labrador sea water formation the 8 2 ka event is the largest climatic signal of our present interglacial with a widespread cooling in the north atlantic region about 8200 years before present it coincides with a meltwater outburst from north american proglacial lakes that is believed to have weakened the atlantic meridional overturning circulation and northward heat transport followed by a recovery of the deep ocean circulation and rising temperatures after a few centuries marine proxy data however date the onset of deep water formation in labrador sea to the same time the subsequent strengthening of the slope current system created a regional signal recorded as an abrupt and persistent surface temperature decrease although similarities in timing are compelling a mechanism to reconcile these apparently contradictory events was missing our simulations show that an abrupt and persistent strengthening of the atlantic subpolar gyre provides a plausible explanation the intense freshwater pulse triggered a transition of the gyre circulation into a different mode of operation stabilized by internal feedbacks and persistent after the cessation of the perturbation as a direct consequence deep water formation around its center intensifies this corresponds to the modern flow regime and stabilizes the meridional overturning circulation possibly contributing to the holocene s climatic stability

links between salinity variation in the caribbean and north atlantic thermohaline circulation variations in the strength of the north atlantic ocean thermohaline circulation have been linked to rapid climate changes1 during the last glacial cycle through oscillations in north atlantic deep water formation and northward oceanic heat flux2 3 4 the strength of the thermohaline circulation depends on the supply of warm salty water to the north atlantic which after losing heat to the atmosphere produces the dense water masses that sink to great depths and circulate back south2 here we analyse two caribbean sea sediment cores combining mg ca palaeothermometry with measurements of oxygen isotopes in foraminiferal calcite in order to reconstruct tropical atlantic surface salinity5 6 during the last glacial cycle we find that caribbean salinity oscillated between saltier conditions during the cold oxygen isotope stages 2 4 and 6 and lower salinities during the warm stages 3 and 5 covarying with the strength of north atlantic deep water formation7 at the initiation of the bolling allerod warm interval caribbean surface salinity decreased abruptly suggesting that the advection of salty tropical waters into the north atlantic amplified thermohaline circulation and contributed to high latitude warming

north atlantic climate response to lake agassiz drainage at coarse and ocean eddy permitting resolutions abstractthe north atlantic climate response to the catastrophic drainage of proglacial lake agassiz into the labrador sea is analyzed with coarse and ocean eddy permitting versions of a global coupled climate model the north atlantic climate response is qualitatively consistent in that a large scale cooling is simulated regardless of the model resolution or region of freshwater discharge however the magnitude and duration of the north atlantic climate response is found to be sensitive to model resolution and the location of freshwater forcing in particular the long term entrainment of freshwater along the boundary at higher resolution and its gradual partially eddy driven escape into the interior leads to low salinity anomalies persisting in the subpolar atlantic for decades longer as a result the maximum decline of the atlantic meridional overturning circulation amoc and the ocean meridional heat transport mht is amplified by about a factor of 2 at ocean eddy permitting resolution and the re

10be dating of former glacial lake naskaupi quebec labrador and timing of its discharges during the last deglaciation abstract the last deglaciation of the laurentide ice sheet in northern quebec and labrador led to the formation of several glacial lakes that drained into ungava bay and the nearby labrador sea assessing the potential impact of their drainage on ocean surface conditions and climate however is limited by the few existing age constraints on ice retreat and associated evolution of these lakes here we report 21 10be ages from shorelines and an outburst flood landform formed by lake naskaupi one of the largest glacial lakes in this region the results indicate that the lake drained from its full extent at 8300 300 a suggesting that it may have contributed to the freshwater forcing initiated by the drainage of lake agassiz ojibway that caused the 8 2 ka cold event additionally the results provide important constraints on the position of the ice margin of the labrador sector during its retreat across this region

the 8 2 ka cooling event caused by laurentide ice saddle collapse abstract the 8 2 ka event was a period of abrupt cooling of 1 3 c across large parts of the northern hemisphere which lasted for about 160 yr the original hypothesis for the cause of this event has been the outburst of the proglacial lakes agassiz and ojibway these drained into the labrador sea in 0 5 5 yr and slowed the atlantic meridional overturning circulation thus cooling the north atlantic region however climate models have not been able to reproduce the duration and magnitude of the cooling with this forcing without including additional centennial length freshwater forcings such as rerouting of continental runoff and ice sheet melt in combination with the lake release here we show that instead of being caused by the lake outburst the event could have been caused by accelerated melt from the collapsing ice saddle that linked domes over hudson bay in north america we forced a general circulation model with time varying meltwater pulses 100 300 yr that match observed sea level change designed to represent the hudson bay ice saddle collapse a 100 yr long pulse with a peak of 0 6 sv produces a cooling in central greenland that matches the 160 yr duration and 3 c amplitude of the event recorded in ice cores the simulation also reproduces the cooling pattern amplitude and duration recorded in european lake and north atlantic sediment records such abrupt acceleration in ice melt would have been caused by surface melt feedbacks and marine ice sheet instability these new realistic forcing scenarios provide a means to reconcile longstanding mismatches between proxy data and models allowing for a better understanding of both the sensitivity of the climate models and processes and feedbacks in motion during the disintegration of continental ice sheets

physical experiments of transpressional folding abstract in order to understand the process of folding in obliquely convergent settings we formed folds within a shear box capable of creating homogeneous transpressional deformations folds were created in a single layer of stiff mixed plasticine and silicone that overlay a newtonian silicone for a variety of plate convergence angles as small amplitude folds became visible they were parallel to the long axis of the horizontal finite strain ellipse with increasing deformation the fold hinges rotated parallel with the long axis of the horizontal finite strain ellipse for all angles of convergence this parallelism indicates that fold hinges once formed rotate with the horizontal strain ellipse rather than as material lines the experiments highlight several interesting effects of transpression dynamics the fold hinges initiate parallel to either 1 or 2 and are parallel to either s 1 or s 2 with increasing deformation neither infinitesimal strain stress nor finite strain is resolvable solely from fold geometry further the net amount of contraction determined by folding across the zone was overestimated in all cases except pure contraction this effect is obvious for the case of wrenching where folding implies that the zone contracts if elongation parallel to the fold hinge is not considered therefore attempts to balance cross sections in transpressional zones will tend to overestimate contraction unless the wrench component of deformation is addressed this result is validated by applying the modeling results in folding in central california adjacent to the san andreas fault where cross section balancing indicates higher amounts of contraction than predicted by plate motion

model support for forcing of the 8 2 ka event by meltwater from the hudson bay ice dome previous model experiments of the 8 2 ka event forced by the drainage of lake agassiz often do not produce climate anomalies as long as those inferred from proxies in addition to the agassiz forcing there is new evidence for significant amounts of freshwater entering the ocean at 8 2 ka from the disintegration of the laurentide ice sheet lis we use the community climate system model version 3 ccsm3 to test the contribution of this additional meltwater flux similar to previous model experiments we find that the estimated freshwater forcing from lake agassiz is capable of sustaining ocean and climate anomalies for only two to three decades much shorter than the event duration of 150 years in proxies using new estimates of the lis freshwater flux 0 13 sv for 100 years from the collapse of the hudson bay ice dome in addition to the agassiz drainage the ccsm3 generates climate anomalies with a magnitude and duration that match within error those from proxies this result is insensitive to the duration of freshwater release a major uncertainty if the total volume remains the same an analysis of the modeled north atlantic freshwater budget indicates that the agassiz drainage is rapidly transported out of the north atlantic while the lis contribution generates longer lasting freshwater anomalies that are also subject to recirculation by the subtropical gyre back into the north atlantic thus the meltwater flux originating from the lis appears to be more important than the agassiz drainage in generating 8 2 ka climate anomalies and is one way to reconcile some model data discrepancies

freshwater outbursts to the oceans from glacial lake agassiz and their role in climate change during the last deglaciation abstract lake agassiz was the largest lake in north america during the last deglaciation as the laurentide ice sheet lis retreated large volumes of water stored in this proglacial lake were episodically released into the oceans these waters were variably routed to the gulf of mexico arctic ocean north atlantic ocean and hudson bay during this period the three largest cooling events in the northern hemisphere closely followed 4 of the 5 largest outbursts from lake agassiz 1 the younger dryas which was preceded by a release of 9500 km 3 2 the preboreal oscillation preceded by releases of 9300 km 3 and 5900 km 3 and 3 the 8 2 ka cold event preceded by a 163 000 km 3 outburst these are respectively fluxes of 0 30 sv 0 29 sv 0 19 sv and 5 2 sv if released in 1 year because the influx of freshwater reaching the north atlantic ocean can inhibit thermohaline circulation partly depending on whether the ocean was in a glacial interglacial or transitional mode of circulation we believe that at least these large outbursts from lake agassiz may have provided the triggers for changes in ocean circulation and in turn for widespread climate change

impact of abrupt sea ice loss on greenland water isotopes during the last glacial period greenland ice cores provide excellent evidence of past abrupt climate changes however there is no universally accepted theory of how and why these dansgaard oeschger do events occur several mechanisms have been proposed to explain do events including sea ice ice shelf buildup ice sheets atmospheric circulation and meltwater changes do event temperature reconstructions depend on the stable water isotope formula see text o and nitrogen isotope measurements from greenland ice cores interpretation of these measurements holds the key to understanding the nature of do events here we demonstrate the primary importance of sea ice as a control on greenland ice core formula see text o 95 of the variability in formula see text o in southern greenland is explained by do event sea ice changes our suite of do events simulated using a general circulation model accurately captures the amplitude of formula see text o enrichment during the abrupt do event onsets simulated geographical variability is broadly consistent with available ice core evidence we find an hitherto unknown sensitivity of the formula see text o paleothermometer to the magnitude of do event temperature increase the change in formula see text o per kelvin temperature increase reduces with do event amplitude we show that this effect is controlled by precipitation seasonality

north atlantic holocene climate evolution recorded by high resolution terrestrial and marine biomarker records abstract holocene climatic change is driven by a plethora of forcing mechanisms acting on different time scales including insolation internal ocean e g atlantic meridional overturning circulation amoc and atmospheric e g north atlantic oscillation nao variability however it is unclear how these driving mechanisms interact with each other here we present five biomarker based paleoclimate records air sea surface temperature and precipitation from a fjordic sediment core revealing north atlantic terrestrial and marine climate in unprecedented detail the early holocene 10 7 7 8 kyrs bp is characterised by relatively high air temperatures while ssts are dampened by melt water events and relatively low precipitation the middle holocene 7 8 3 2 kyrs bp is characterised by peak ssts declining air temperatures and high precipitation a pronounced marine thermal maximum occurs between 7 5 5 kyrs bp 3000 years after the terrestrial thermal maximum driven by melt water cessation and an accelerating amoc the neoglacial cooling between 5 8 and 3 2 kyrs bp leads into the late holocene we demonstrate that an observed modern link between icelandic precipitation variability during different nao phases may have existed from 7 5 kyrs bp a simultaneous decoupling of both air and sea surface temperature records from declining insolation at 3 2 kyrs bp may indicate a threshold after which internal feedback mechanisms namely the nao evolved to be the primary drivers of icelandic climate on centennial time scales

paleohydraulics of the last outburst flood from glacial lake agassiz and the 8200 bp cold event abstract during the last deglaciation of north america huge proglacial lakes formed along the southern margin of the laurentide ice sheet the largest of these was glacial lake agassiz which formed about 11 7 14 c kyr and drained into hudson bay about 7 7 14 c kyr 8 45 cal kyr overflow from these lakes was variably directed to the mississippi st lawrence and mackenzie drainage systems and it is thought that switches in routing were accompanied by a response in ocean circulation that produced abrupt climate events when the ice dam across hudson bay finally was breached a massive flood drained lake agassiz which was routed through hudson strait to the labrador sea in terms of stored water volume the largest reservoir was associated with the kinojevis level of lake agassiz for this maximum filling the impounded water volume available to produce floods is estimated as 40 000 151 000 km 3 depending on the location of the ice margin and route used the timing of this rapid release of stored freshwater just precedes the early holocene cooling event at 8 2 cal kyr bp we use the spring hutter theory to simulate flood hydrographs for floods that originate in subglacial drainage conduits and find that flood magnitude and duration are 5 sv and 0 5 yr multiple fillings and floods are possible as are single floods having a complex multipulse structure modelling results suggest that the outburst flood from lake agassiz may have terminated before the lake surface elevation dropped to sea level and that the flood ended when a stable drainage channel was established connecting lake agassiz to the tyrrell sea

centennial scale climate cooling with a sudden cold event around 8 200 years ago the extent of climate variability during the current interglacial period the holocene is still debated temperature records derived from central greenland ice cores show one significant temperature anomaly between 8 200 and 8 100 years ago which is often attributed to a meltwater outflow into the north atlantic ocean and a slowdown of north atlantic deep water formation this anomaly provides an opportunity to study such processes with relevance to present day freshening of the north atlantic anomalies in climate proxy records from locations around the globe are often correlated with this sharp event in greenland but the anomalies in many of these records span 400 to 600 years start from about 8 600 years ago and form part of a repeating pattern within the holocene more sudden climate changes around 8 200 years ago appear superimposed on this longer term cooling the compounded nature of the signals implies that far field climate anomalies around 8 200 years ago cannot be used in a straightforward manner to assess the impact of a slowdown of north atlantic deep water formation and the geographical extent of the rapid cooling event 8 200 years ago remains to be determined

high resolution isotope records of early holocene rapid climate change from two coeval stalagmites of katerloch cave austria abstract two coeval stalagmites from katerloch cave show pronounced intervals of low 18o values around 8 2 9 1 and 10 0 kyr all ages are reported before the year 2000 ad and represent an inorganic u th dated climate archive from the southeast of the european alps a region where only very few well dated climate records exist the o isotope curves providing near annual resolution allow a direct comparison to the greenland ice core records as temperature was the primary factor controlling the o isotopic composition of katerloch speleothems the 8 2 kyr climate anomaly lasted about one century from 8196 to 8100 yr with a maximum amplitude of 1 1 at 8175 yr the event is characterized by a rapid onset and a more gradual demise and u th data as well as annual lamina counting support a rapid climate change towards cooler conditions within 10 20 yr there is no strong evidence that the 8 2 kyr anomaly was superimposed on a pronounced longer term cooling episode nor do the new data support two separate cooling events within the 8 2 kyr event as reported by other studies our record also shows a distinct climate anomaly around 9 1 kyr which lasted 70 110 yr and showed a maximum amplitude of 1 0 i e it had a similar duration and amplitude as the central 8 2 kyr event compared to the 8 2 kyr event the 9 1 kyr anomaly shows a more symmetrical structure but onset and demise still occurred within a few decades only the different progression of the 8 2 asymmetrical and 9 1 kyr anomaly symmetrical suggests a fundamental difference in the trigger and or the response of the climate system moreover both stalagmites show evidence of a climate anomaly around 10 0 kyr which was of comparable magnitude to the two subsequent events using a well constrained modern calibration between air temperature and 18o of precipitation for the study area and cave monitoring data confirming speleothem deposition in katerloch reflecting cave air temperature a maximum cooling by ca 3 c can be inferred at 8 2 and 9 1 kyr which is similar to other estimates e g from lake ammersee north of the alps the o isotopic composition of meteoric precipitation however is a complex tracer of the hydrological cycle and these temperature estimates do not take into account additional effects such as changes in the source area or synoptic shifts apart from that the relative thickness of the seasonally controlled lamina types in the katerloch stalagmites remains rather constant across the intervals comprising the isotopic anomalies i e the stalagmite petrography argues against major shifts in seasonality during the early holocene climate excursions

the 8 2 kyr bp event simulated by a global atmosphere sea ice ocean model seven freshwater perturbation experiments were performed with a global atmosphere sea ice ocean model to study the mechanism behind the 8 2 kyr bp holocene cooling event these experiments differed in initial state and duration of the applied freshwater pulse while the amount of freshwater was kept constant 4 67x10 14 m 3 one of the scenarios with freshwater added to the labrador sea at a rate of 0 75 sv during 20 years resulted in weakening of the north atlantic thermohaline circulation during 320 years and surface cooling varying from 1 to 5 degreesc over adjacent continents this result is consistent with proxy data suggesting that a meltwater induced weakening of the thermohaline circulation caused the event moreover our results indicate that the time scale of the meltwater release and the initial state are important as both have a strong effect on the magnitude and duration of the produced model response

the greenland ice sheet during the last glacial cycle current ice loss and contribution to sea level rise from a palaeoclimatic perspective abstract during the last interglacial lig period between 130 and 116 thousand years before present ka bp the greenland ice sheet gris was considerably reduced in size contributing to a global mean sea level msl rise of 0 5 4 2 m relative to the present this is not sufficient to explain the 6 9 m msl rise estimated for the lig which implies that a significant contribution to the lig highstand came from antarctica following the lig the gris grew and attained its maximum volume of about 12 m global sea level equivalents sles between 18 and 16 ka bp since then the gris margins have retreated on an order of several hundred km following a general pattern of lagged response to changes in high latitude summer insolation and global greenhouse gas concentrations on shorter timescales and over smaller spatial scales the combination of internal ice sheet dynamics and external climate dynamics has often resulted in a more complex and asynchronous behaviour of the ice sheet margin the gris probably reached its minimum holocene extent around 4 ka bp and modelling studies suggest that it contributed to a rise in global msl of less than 0 2 m relative to the present at this time a period of steady growth followed the holocene minimum and in many areas the ice sheet advanced beyond its present limits during the little ice age i e the last few centuries currently the gris occupies an area of 1 7 10 6 km 2 and features a volume of 2 96 10 6 km 3 which amounts to 7 4 m sle observations show that the rate of mass loss from the gris has been accelerating over the past few decades and ice sheet modelling indicates that we have to go back to the last deglaciation around 10 ka bp in order to find sustained melt rates that were higher than those experienced over the last decade the future development of the gris will have a profound influence on sea level both globally and regionally but there are large uncertainties related to how the gris will respond to future global warming based on a range of modelling studies employing different emission scenarios the gris is expected to contribute about 0 1 0 3 m to global msl rise by the end of the 21st century in a longer time perspective modelling suggests that melting of the gris may replace ocean thermal expansion as the most important factor in future sea level rise potentially contributing with 0 7 2 6 m sle within the next 500 years multi millennial simulations show that the entire ice sheet 7 4 m sle might disappear completely within less than three thousand years under high emission scenarios i e with atmospheric co 2 reaching four times preindustrial levels

a high resolution study of holocene paleoclimatic and paleoceanographic changes in the nordic seas 1 high resolution records from images core md95 2011 in the eastern norwegian sea provide evidence for relatively large and small scale high latitude climate variability throughout the holocene during the early and mid holocene a situation possibly driven by consistent stronger westerlies increased the eastward influence of arctic intermediate and near surface waters for the late holocene a relaxation of the atmospheric forcing resulted in increased influence of atlantic water the main changes in holocene climate show no obvious connection to changing solar irradiance and spectral analysis reveals no consistent signature for any periodic behavior of holocene climate at millennial or centennial timescales there are however indications of consistent multidecadal variability

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

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

links between early holocene ice sheet decay sea level rise and abrupt climate change the beginning of the holocene interglacial was marked by ice sheet melting and sea level rise a review of sea level and climate records identifies two sea level jumps associated with the final drainage of glacial lake agassiz and links them to an abrupt cooling event

fingerprinting the 8 2 ka bp event climate response in a coupled climate model using results from coupled climate model simulations of the 8 2 ka climate event that produced a cold period over greenland in agreement with the reconstructed cooling from ice cores we investigate the typical pattern of climate anomalies fingerprint to provide a framework for the interpretation of global proxy data for the 8 2 ka climate event for this purpose we developed an analysis method that isolates the forced temperature response and provides information on spatial variations in magnitude timing and duration that characterise the detectable climate event in proxy archives our analysis shows that delays in the temperature response to the freshwater forcing are present mostly in the order of decades 30 a over central greenland the north atlantic ocean initially cools in response to the freshwater perturbation followed in certain parts by a warm response this delay occurring more than 200 a after the freshwater pulse hints at an overshoot in the recovery from the freshwater perturbation the south atlantic and the southern ocean show a warm response reflecting the bipolar seesaw effect the duration of the simulated event varies for different areas and the highest probability of recording the event in proxy archives is in the north atlantic ocean area north of 40 n our results may facilitate the interpretation of proxy archives recording the 8 2 ka event as they show that timing and duration cannot be assumed to correspond with the timing and duration of the event as recorded in greenland ice cores copyright 2011 john wiley sons ltd

subtropical atlantic salinity variability and atlantic meridional circulation during the last deglaciation during the last deglaciation ca 21 10 ka freshening of the north atlantic surface likely caused reductions in atlantic meridional overturning circulation amoc the mechanisms related to amoc recovery remain poorly understood here we present three new deglacial surface temperature and 18oseawater 18osw reconstructions from the western subtropical north and south atlantic similarities to tropical caribbean and western atlantic 18osw records suggest that a salty surface water mass accumulated in the western atlantic from 27 s to 33 n during periods of reduced amoc however 18osw decreases led deep amoc resumption by hundreds of years we suggest that the northward export of salt previously trapped in the western atlantic resulted in the early establishment of a shallow overturning circulation that eventually culminated in deep amoc resumption implying that amoc may constitute a self limiting system

nd isotopic structure of the pacific ocean 40 10 ma and evidence for the reorganization of deep north pacific ocean circulation between 36 and 25 ma abstract early paleogene neodymium nd isotopic reconstructions of ocean circulation in the pacific suggest a bipolar mode of overturning circulation characterized by convection of deep water masses from 70 to at least 40 ma this mode is fundamentally different from the sluggish diffusive mode that characterizes the modern north pacific therefore major reorganization of pacific circulation must have occurred between 40 ma and the modern to investigate the potential timing of such a reorganization we present new nd isotope data from drill sites spanning 40 to 10 ma here we use nd isotopic analyses to identify the sources of dissolved nd to the pacific in order to evaluate the application of the nd proxy for ancient water mass composition we combine analyses of fossil fish debris sequentially leached oxide coatings and leach residues to further refine our understanding of nd marine geochemical cycling and to contribute new constraints on the paleoceanographic history of the pacific overall these data support previous interpretations of the convection of deep water in the pacific sector of the southern ocean and surface ventilated north pacific deep water formation from 70 to 40 ma the new nd isotope data suggest a gradual transition between 36 and 25 ma a time period characterized by global cooling this transition consists of a change from north pacific nd isotopic signatures controlled by convection and water mass mixing to values heavily influenced by overprinting from other sources of nd to the deep pacific such a transition is consistent with a change in overturning circulation from a paleogene mode to a mode dominated by sluggish diffusive mixing after 25 ma the data indicate a significant contribution of radiogenic dissolved nd to the deep waters of the north pacific the most likely source of radiogenic dissolved nd would have been labile volcanic ash that altered the composition of the water mass as it slowly advected through the north pacific a slowing of overturning circulation in the north pacific could have resulted in diminished poleward heat transport increasing equator to pole thermal gradients and contributing to a cooling global climate

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

deglaciation of the greenland and laurentide ice sheets interrupted by glacier advance during abrupt coolings abstract understanding patterns of ice sheet deglaciation is key for predicting the rate of future ice sheet melt yet the processes underlying deglaciation remain elusive the early holocene 11 7 ka to 8 2 ka greenlandian stage represents the most recent period when the laurentide and greenland ice sheets underwent large scale recession moreover this ice sheet recession occurred under the backdrop of regional temperatures that were similar to or warmer than today and comparable to those projected for the upcoming centuries reconstructing laurentide and greenland ice sheet behavior during the early holocene and elucidating the mechanisms dictating this behavior may serve as a partial analog for future greenland ice sheet change in a warming world here we present 123 new 10be surface exposure ages from two sites on baffin island and southwestern greenland that constrain the behavior of the laurentide and greenland ice sheets and an independent alpine glacier during the early holocene on baffin island we focus on a unique area where moraines deposited by the laurentide ice sheet rest directly adjacent to moraines deposited by an independent alpine glacier sixty one 10be ages reveal that advances and or stillstands of the laurentide ice sheet and an alpine glacier occurred in unison around 11 8 ka 10 3 ka and 9 2 ka sixty two 10be ages from southwestern greenland indicate that the greenland ice sheet margin experienced re advances or stillstands around 11 6 ka 10 4 ka 9 1 ka 8 1 ka and 7 3 ka our results reveal that ice sheets respond to climate perturbations on the same centennial timescale as small alpine glaciers we hypothesize that during the warming climate of the early holocene freshening of the north atlantic ocean induced by ice sheet melt resulted in regional cooling and brief periods of ice sheet stabilization superimposed on net glacier recession these observations point to a negative feedback mechanism inherent to melting ice sheets in the baffin bay region that slows ice sheet recession during intervals of otherwise rapid deglaciation

ensemble water isotope enabled coupled general circulation modeling insights into the 8 2 ka event 1 freshwater forcing has long been postulated as a catalyst for abrupt climate change because of its potential to interfere with thermohaline circulation thc the most recent example may have occurred about 8 2 ka ago with the sudden drainage of glacial lakes agassiz and ojibway into the hudson bay we perform an ensemble of simulations for this freshwater release using the fully coupled atmosphere ocean general circulation model goddard institute for space studies modele r in all cases simulated effects include reduced ocean heat transport and enhanced atmospheric heat transport in the atlantic increased surface albedo through greater low cloud and sea ice cover and local cooling of up to 3 c our suite of ensemble experiments allows us to examine the importance of the initial ocean state in particular the presence or absence of labrador sea water in controlling the magnitude and length of the climate response water isotope tracers included in this model provide an improved means for direct comparisons of the modeled tracer response to water isotope based climate proxy data comparison of model simulations to data implies that there was an abrupt approximate halving of atlantic thc hence providing strong support for the hypothesis that the 8 2 ka event was caused by an abrupt release of fresh water into the north atlantic

the 8 2 ka bp event in north eastern north america first combined oxygen and hydrogen isotopic data from peat in newfoundland finding direct evidence for atmospheric circulation change in terrestrial records of holocene climate variability remains a fundamental challenge here we present the first combined stable oxygen and hydrogen isotopic palaeorecord from a peatland core in newfoundland canada sphagnum cellulose samples were isolated from a core from nordan s pond bog newfoundland and analysed for d values combined with existing 18o data the resulting d 18o bi plot correlates directly with existing measurements of the modern late 20th century isotopic composition of precipitation from gnip stations in nova scotia and labrador implying a close relationship between the estimated isotopic composition of source water used by the mosses and that of the source precipitation we use the relative variations between the two isotope records to test the hypothesis that atmospheric circulation changed in the millennium following the 8 2 ka bp climate event the data reveal a secondary complex isotopic response 200 years 8250 8050 a bp after a primary oxygen isotopic event that is widespread in the north atlantic region this secondary event is characterized by a divergence in oxygen and hydrogen isotope records that can most plausibly be explained by the augmentation of precipitation moisture from a more distant and more continental vapour source

timing and duration of north american glacial lake discharges and the younger dryas climate reversal abstract radiocarbon dated sediment cores from the champlain valley northeastern usa contain stratigraphic and micropaleontologic evidence for multiple high magnitude freshwater discharges from north american proglacial lakes to the north atlantic of particular interest are two large closely spaced outflows that entered the north atlantic ocean via the st lawrence estuary about 13 200 12 900 cal yr bp near the beginning of the younger dryas cold event we estimate from varve chronology sedimentation rates and proglacial lake volumes that the duration of the first outflow was less than 1 yr and its discharge was approximately 0 1 sv 1 sverdrup 106 m3 s 1 the second outflow lasted about a century with a sustained discharge sufficient to keep the champlain sea relatively fresh for its duration according to climate models both outflows may have had sufficient discharge duration and timing to affect meridional ocean circulation and climate in this report we compare the proglacial lake discharge record in the champlain and st lawrence valleys to paleoclimate records from greenland ice cores and cariaco basin and discuss the two step nature of the inception of the younger dryas

surface and deep ocean interactions during the cold climate event 8200 years ago evidence from a north atlantic deep sea sediment core reveals that the largest climatic perturbation in our present interglacial the 8200 year event is marked by two distinct cooling events in the subpolar north atlantic at 8490 and 8290 years ago an associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the atlantic meridional overturning circulation the existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying laurentide ice sheet margin

magnetic parameters and their palaeoclimatic implications the sediment record of the last 15 500 cal bp from laguna potrok aike argentina fil irurzun maria alicia universidad nacional del centro de la provincia de buenos aires facultad de ciencias exactas instituto de fisica arroyo seco argentina consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico tandil centro de investigaciones en fisica e ingenieria del centro de la provincia de buenos aires argentina

evidence for higher than average air temperatures after the 8 2 ka event provided by a central european δ 18 o record the so called 8 2 ka event represents one of the most prominent cold climate anomalies during the holocene warm period accordingly several studies have addressed its trigger mechanisms absolute dating and regional characteristics so far however knowledge about subsequent climate recovery is still limited although this might be essential for the understanding of rapid climatic changes here we present a new sub decadally resolved and precisely dated oxygen isotope o record for the interval between 7 7 and 8 7 ka bp 10 calendar years before ad 1950 derived from the calcareous valves of benthic ostracods preserved in the varved lake sediments of pre alpine mondsee austria besides a clear reflection of the 8 2 ka event showing a good agreement in timing duration and magnitude with other regional stable isotope records the high resolution mondsee lake sediment record provides evidence for a 75 year long interval of higher than average o values directly after the 8 2 ka event possibly reflecting increased air temperatures in central europe this observation is consistent with evidence from other proxy records in the north atlantic realm thus most probably reflecting a hemispheric scale climate signal rather than a local phenomenon as a possible trigger we suggest an enhanced resumption of the atlantic meridional overturning circulation amoc supporting assumptions from climate model simulations

lateral fold growth and fold linkage we use a three dimensional finite element model for viscous flow to investigate the lateral linkage mode of two initially isolated fold segments depending on the separation distance between the fold hinge lines four different fold linkage scenarios are observed linear linkage yields a sub cylindrical fold with a saddle at the location where the two initial folds linked oblique linkage produces a fold resembling a type ii refold structure oblique no linkage results in two curved folds with fold axes plunging in opposite directions linear no linkage yields a fold train of two separate subcylindrical folds with fold axes plunging in opposite directions representative numerical models for detachment and matrix folding for both power law and linear viscous rheologies exhibit the same four fold linkage modes the transition from linkage to no linkage occurs when the fold separation between the initially isolated folds is slightly larger than one half of the low amplitude fold wavelength we demonstrate a natural example of lateral fold linkage from the kurdistan region of iraq and point out the economic interest of saddle points linking the initially isolated fold segments

reconstructing paleosalinity from δ18o coupled model simulations of the last glacial maximum last interglacial and late holocene reconstructions of salinity are used to diagnose changes in the hydrological cycle and ocean circulation a widely used method of determining past salinity uses oxygen isotope ow residuals after the extraction of the global ice volume and temperature components this method relies on a constant relationship between ow and salinity throughout time here we use the isotope enabled fully coupled general circulation model gcm hadcm3 to test the application of spatially and time independent relationships in the reconstruction of past ocean salinity simulations of the late holocene lh last glacial maximum lgm and last interglacial lig climates are performed and benchmarked against existing compilations of stable oxygen isotopes in carbonates oc which primarily reflect ow and temperature we find that hadcm3 produces an accurate representation of the surface ocean oc distribution for the lh and lgm our simulations show considerable variability in spatial and temporal ow salinity relationships spatial gradients are generally shallower but within 50 of the actual simulated lh to lgm and lh to lig temporal gradients and temporal gradients calculated from multi decadal variability are generally shallower than both spatial and actual simulated gradients the largest sources of uncertainty in salinity reconstructions are found to be caused by changes in regional freshwater budgets ocean circulation and sea ice regimes these can cause errors in salinity estimates exceeding 4 psu our results suggest that paleosalinity reconstructions in the south atlantic indian and tropical pacific oceans should be most robust since these regions exhibit relatively constant ow salinity relationships across spatial and temporal scales largest uncertainties will affect north atlantic and high latitude paleosalinity reconstructions finally the results show that it is difficult to generate reliable salinity estimates for regions of dynamic oceanography such as the north atlantic without additional constraints

effects of glacial meltwater in the giss coupled atmosphereocean model 1 north atlantic deep water response varying magnitudes of freshwater input through the st lawrence are added to different versions of the giss coupled atmosphere ocean model the results show a generally linear response in percentage north atlantic deep water nadw reduction to the volume of water added without any obvious threshold effects when the estimated best guess freshwater input for the allerod younger dryas interval is added only small reductions in nadw production occur in less than a century with complete cessation requiring more than 300 years if the ocean circulation was weaker in the allerod preceding the younger dryas somewhat larger percentage reductions occur and it would take 150 200 years for a complete shutdown the freshening is aided by a positive feedback associated with the reduction in evaporation relative to precipitation in the north atlantic amounting to some 20 30 of the added freshwater runoff when the freshwater input is stopped after complete nadw shutdown nadw production does not resume with complete nadw cessation cooling eventually occurs throughout the northern hemisphere including regions where it is not thought to have arisen during the younger dryas and the globe cools by 1 c due primarily to the sea ice albedo feedback while in many regions the observed cooling is reproduced the cooling in greenland is much less than is estimated with only 50 nadw cessation cooling is primarily around the north atlantic no rapid warming is seen in response to freshwater cessation the sensitivity displayed here for nadw reduction due to freshwater input from the st lawrence is roughly similar to that associated with increased precipitation accompanying global warming in the next century so a similar sensitivity may arise with younger dryas freshwater inputs from other suspected locations greenland europe

a transpressional origin for the west spitsbergen fold and thrust belt insight from analog modeling 1 the west spitsbergen fold and thrust belt formed along the transform plate boundary between greenland and the western barents sea during paleocene eocene breakup in the northern north atlantic approximately 20 40 km margin perpendicular shortening accumulated in the belt has been attributed to transpression and strain partitioning in a restraining bend but also to head on collision we have applied scaled analog tectonic modeling to test the former hypothesis a pack of brittle quartz sand with a shallow thin layer of silicon putty was deformed by transpression at a 15 convergence angle by movement of a basal plate the kinematics was quantified by means of digital particle image velocimetry the result was a doubly vergent transpressional wedge consisting of a steeply tapered retrowedge and a strongly internally deformed steeply tapered prowedge separated by a central strike slip zone and an adjacent low taper thin skinned fold and thrust belt the doubly vergent wedge evolved through several kinematic phases three main stages were identified namely 1 initial distributed deformation 2 development of an oblique doubly vergent wedge with progressive evolution of local strain partitioning along the marginal shear zones and finally 3 a stage of full strain partitioning between a central strike slip zone and reverse displacement along marginal shear zones with folding and thrusting in a thin skinned belt on the proside the analog model convincingly reproduced the geometry and the kinematic evolution of the west spitsbergen fold and thrust belt supporting the hypothesis of its formation by strain partitioning in transpression with a small angle of convergence and significant lateral displacement