early jurassic late pliensbachian co2 concentrations based on stomatal analysis of fossil conifer leaves from eastern australia the stomatal index a measure of stomatal density of an extinct australian early jurassic araucariacean conifer species allocladus helgei jansson is used to reconstruct the atmospheric carbon dioxide concentration pco 2 in the early jurassic the fossil leaves are preserved in a single bed palynologically dated to late pliensbachian similar to 185 183 mya atmospheric pco 2 is estimated from the ratios between the stomatal index of a helgei and the stomatal indices of three modern analogs nearest living equivalent plants co2 concentration in the range of similar to 750 975 ppm was calibrated from the fossil material with a best estimated mean of similar to 900 ppm the new average pco 2 determined for the late pliensbachian is thus similar to although similar to 10 lower than previously inferred minimum concentrations of similar to 1000 based on data from the northern hemisphere but may help constrain pco 2 during this period our results are the first pco 2 estimates produced using jurassic leaves from the southern hemisphere and show that i paleo atmospheric pco 2 estimates are consistent at a global scale though more investigations of southern hemisphere material are required and ii the stomatal proxy method can now be used without the context of relative change in pco 2 when applying the correct methodology c 2014 the authors published by elsevier b v on behalf of international association for gondwana research less

millennial scale fluctuations of the european ice sheet at the end of the last glacial and their potential impact on global climate abstract reconstructing northern hemisphere ice sheet oscillations and meltwater routing to the ocean is important to better understand the mechanisms behind abrupt climate changes to date research efforts have mainly focused on the north american laurentide ice sheets lis leaving the potential role of the european ice sheet eis and of the scandinavian ice sheet sis in particular largely unexplored using neodymium isotopes in detrital sediments deposited off the channel river we provide a continuous and well dated record for the evolution of the eis southern margin through the end of the last glacial period and during the deglaciation our results reveal that the evolution of eis margins was accompanied with substantial ice recession especially of the sis and simultaneous release of meltwater to the north atlantic these events occurred both in the course of the eis to its lgm position i e during heinrich stadial hs 3 and hs2 31 29 ka and 26 23 ka respectively and during the deglaciation i e at 22 ka 20 19 ka and from 18 2 0 2 to 16 7 0 2 ka that corresponds to the first part of hs1 the deglaciation was discontinuous in character and similar in timing to that of the southern lis margin with moderate ice sheet retreat from 22 5 0 2 ka in the baltic lowlands as soon as the northern summer insolation increase from 23 ka and an acceleration of the margin retreat thereafter from 20 ka importantly our results show that eis retreat events and release of meltwater to the north atlantic during the deglaciation coincide with amoc destabilisation and interhemispheric climate changes they thus suggest that the eis together with the lis could have played a critical role in the climatic reorganization that accompanied the last deglaciation finally our data suggest that meltwater discharges to the north atlantic produced by large scale recession of continental parts of northern hemisphere ice sheets during hs could have been a possible source for the oceanic perturbations i e amoc shutdown responsible for the marine based ice stream purge cycle or so called he s that punctuate the last glacial period

an abrupt change in the african monsoon at the end of the younger dryas 1 high resolution studies of variations in the elemental and stable carbon and nitrogen isotope composition of organic matter in cores from lakes malawi tanganyika and bosumtwi tropical africa indicate an abrupt change in the wind driven circulation of these lakes that within the limits of available chronologies was contemporaneous with the end of the younger dryas in the northern hemisphere the change was also coincident with shifts in surface winds recorded in cores from off the west and northeast coasts of africa a range of other proxies indicate that these changes in wind regime were accompanied by a marked increase in precipitation in the northern tropics africa south of 5 10 s on the other hand initially suffered drought conditions together the evidence suggests an abrupt northward translation of the african monsoon system at circa 11 5 0 25 ka b p the data assembled here contribute to a growing body of work showing that the younger dryas was a major climatic excursion in tropical africa furthermore they add substance to recent suggestions that climatic events in the southern hemisphere may have played a significant role in the abrupt demise of the younger dryas

introduction to atlantic meridional overturning circulation amoc a striking conclusion of the intergovernmental panel on climate change 2007 report is the crucial role that the atlantic meridional overturning circulation amoc may play in anthropogenic climate change however these ipcc coupled climate simulations show a broad range of uncertainty in the magnitude and timing of amoc transport change ranging from none to nearly complete collapse within the 21st century the potential consequences of large changes in the characteristics of amoc have motivated the creation in the united states of an interagency program and implementation plan to develop monitoring and prediction capabilities for the amoc this program parallels the development of substantial monitoring efforts by european south american and african countries notably the uk rapid and rapid watch programs the papers contained in this volume are derived from presentations at the first u s atlantic meridional overturning circulation amoc meeting held 4 6 may 2009 to review the us implementation plan and its coordination with other monitoring activities the atlantic meridional overturning circulation consists of multiple components illustrated in an attached figure water enters the south atlantic at upper and intermediate depths through both western and eastern routes where eddy transport is especially important and is transported northward across the equator where it recirculates within the northern subtropical and subpolar gyres the northern end is defined by the sinking regions of the nordic seas and the labrador sea where the waters that eventually form the upper and lower branches of north atlantic deep water are conditioned high surface salinities the result of high net evaporation in the tropics and subtropics including the mediterranean sea and presence of regions of the arctic ocean that remain ice free even in winter allow for the rapid cooling and thus densification of surface water this dense surface water becomes the source of deep water formation in the sinking regions in addition to transporting mass the amoc transports roughly half of the total amount of heat carried northward through the northern subtropics down the temperature gradient by the ocean in contrast in the southern hemisphere amoc transports heat up gradient from the cool circumpolar current to the warm tropics paleoevidence suggests that amoc heat transport in the two hemispheres has varied over time in ways intimately tied to millennial changes in the earth s climate in one example the abrupt younger dryas spell of cold weather over the north atlantic which began 13 000 years ago has generally been linked to a millennial shutdown of the amoc as a result of massive freshwater discharge from the north american continent the current amoc monitoring array consists of a series of instrumented transects located across key passages see cunningham et al 2010 for a recent review in the arctic and sub arctic transects cross fram strait denmark strait and the faroe channel connecting greenland iceland and the united kingdom as well as the entrance to the labrador sea further south and extending outwards from the east coast of north america there are a series of monitoring arrays including arrays of the canadian atlantic zone monitoring program deployments of the rapid western atlantic variability experiment wave line w at 39 n as well as the rapid moc moored array the latter spans the entire atlantic basin along 26 5 n at tropical latitudes we have the meridional overturning variability experiment move array at 16 n while in the southern hemisphere a corresponding basin spanning transect is being established at the latitude of cape of good hope complemented by arrays at drake passage

alternative routing of lake agassiz overflow during the younger dryas new dates paleotopography and a re evaluation abstract overflow from glacial lake agassiz has been implicated in affecting late glacial ocean circulation and climate the timing of the diversion of lake agassiz overflow away from its southern routing to the gulf of mexico correlates closely with the younger dryas ca 11 10 14 c ka new models of paleotopography in the eastern outlet region of lake agassiz adjusted for differential isostatic rebound show that overflow to the superior basin would have occurred when the lake formed its herman norcross and tintah beaches just after 10 9 14 c ka if the laurentide ice sheet lis had retreated from the thunder bay ontario region by that time however the absence of coarse flood deposits normally associated with large overflows plus the morphology of possible overflow channels in the thunder bay region suggest that agassiz overflow may not have been routed into the great lakes during the younger dryas as previously believed in addition non carbonate radiocarbon dates in the thunder bay area are all younger than 10 14 c ka suggesting that this area may not have been deglaciated until after the younger dryas thus preventing lake agassiz overflow through this area an alternative route of overflow from lake agassiz during the younger dryas could have been through the northwestern clearwater outlet to the arctic ocean although all research to date shows that this region was covered by ice until 10 14 c ka on the other hand it is possible that older and coarser overflow sediment may now lie buried beneath finer deposits associated with the 10 14 c ka marquette readvance of the lis which ponded waters in valleys between lake agassiz and lake superior or that eastward overflow in this area was not catastrophic

stomatal proxy record of co2 concentrations from the last termination suggests an important role for co2 at climate change transitions a new stomatal proxy based record of co2 concentrations co2 based on betula nana dwarf birch leaves from the hasseldala port sedimentary sequence in south eastern sweden is presented the record is of high chronological resolution and spans most of greenland interstadial 1 gi 1a to 1c allerod pollen zone greenland stadial 1 gs 1 younger dryas pollen zone and the very beginning of the holocene preboreal pollen zone the record clearly demonstrates that i co2 were significantly higher than usually reported for the last termination and ii the overall pattern of co2 evolution through the studied time period is fairly dynamic with significant abrupt fluctuations in co2 when the climate moved from interstadial to stadial state and vice versa a new loss on ignition chemical record used here as a proxy for temperature lends independent support to the hasseldala port co2 record the large amplitude fluctuations around the climate change transitions may indicate unstable climates and that tippingpoint situations were involved in last termination climate evolution the scenario presented here is in contrast to co2 records reconstructed from air bubbles trapped in ice which indicate lower concentrations and a gradual linear increase of co2 through time the prevalent explanation for the main climate forcer during the last termination being ocean circulation patterns needs to re examined and a larger role for atmospheric co2 considered

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

regional temperature atmospheric circulation and sea ice variability within the younger dryas event constrained using a speleothem from northern iberia precisely dated high resolution stable isotope and trace element data from a stalagmite from la garma cave northern spain reveal several stages of distinct climatic variability along the northern iberian atlantic margin and provide new constraints on the latitude of north atlantic westerlies during the younger dryas event yd westerly wind position reconstructed using sub annually resolved mg data as a proxy for sea spray contributions associated with wind strength at this coastal cave site during the early yd 12 85 12 15 kyr oscillated meridionally resembling the decadal scale component of the modern north atlantic oscillation nao northward repositioning of westerly storm tracks over northern iberia began at 12 15 kyr consistent with other high resolution wind proxy reconstructions from central and northern europe but occurred more gradually nearer the atlantic margin from 12 15 kyr to the yd termination atmospheric circulation resembled a persistently positive nao with westerlies reaching their maximum northward extent at 11 8 kyr reflected by a mg concentration minimum at this time air temperature reflected by our 18o data and iberian wind strength were predominantly coupled throughout the yd suggesting that temperature modulated sea ice extent and consequently controlled westerly wind latitude however our data suggest that abrupt warming at 12 1 kyr was followed by much more gradual northward shifts in westerly position and that a lag existed between the warming and sea ice retreat this gradual return of the westerlies to the north beginning at 12 1 kyr is consistent with inferred changes in wind strength at other european sites additionally atmospheric circulation inferred from our northern iberian wind strength proxy record generally tracked low frequency meridional shifts in intertropical convergence zone position corroborating past research that suggested closely coupled low and high latitude atmospheric teleconnections over this period

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

late pleistocene earthquake triggered moraine dam failure and outburst of lake zurich switzerland 1 lakes impounded by moraines may be considered hazardous in glaciated areas throughout the world because dams can fail suddenly producing destructive floods with peak discharges far in excess of normal flows here we present a comprehensive case study in the zurich switzerland area that reveals several independent lines of evidences for the occurrence of a late pleistocene 13 760 calibrated years b p moraine breach and subsequent lake zurich outburst discharge volume 2 5 km3 we use seismic and core data in order to track and date the geological fingerprint of this event data from areas downstream of the lake show coarse grained massive 25 m thick reworked morainic deposits behind four breached zones in lake zurich sedimentary structures recorded in a paleowater depth of 36 m indicate strong outburst induced currents hydrodynamic calculations reconstructing the sediment transport capacity explaining the observed bed forms allow estimating averaged outburst discharge to exceed minimum values of 2400 m3s 1 the potential maximal magnitude of the outburst is inferred from calculations considering critical flow conditions through the breaches revealing estimated peak discharge of 20 600 m3s 1 we also discuss long term causes and short term trigger mechanisms of the dam failure that occurred several thousand years after moraine formation and show that it coincides with a reconstructed strong northern alpine earthquake magnitude 6 5 our data thus suggest that catastrophic drainage of lake zurich was initiated as a consequence of the moraine dam failure that either was triggered by primary earthquake shaking or by secondary effects such as overtopping by landslide generated waves

estimated strength of the atlantic overturning circulation during the last deglaciation the atlantic meridional overturning circulation affects the latitudinal distribution of heat and is a key component of the climate system proxy reconstructions based on sedimentary 231 pa 230 th ratios and the difference between surface and deep water radiocarbon ages indicate that during the last glacial period the overturning circulation was reduced during millennial scale periods of cooling 1 5 however much debate exists over the robustness of these proxies 6 8 here we combine proxy reconstructions of sea surface and air temperatures and a global climate model to quantitatively estimate changes in the strength of the atlantic meridional overturning circulation during the last glacial period we find that relative to the last glacial maximum the overturning circulation was reduced by approximately 14 sv during the cold heinrich event 1 during the younger dryas cold event the overturning circulation was reduced by approximately 12 sv relative to the preceding warm interval these changes are consistent with qualitative estimates of the overturning circulation from sedimentary 231 pa 230 th ratios in addition we find that the strength of the overturning circulation during the last glacial maximum and the holocene epoch are indistinguishable within the uncertainty of the reconstruction the atlantic meridional overturning circulation amoc is an important feature of the earth s climate system it is characterized by northward flowing water masses at the surface ocean deepwater formation in the north atlantic and southward flowing water masses in the intermediate to deep ocean this overturning cell is complemented by an underlying generally weaker reversed overturning cell that originates in the southern ocean in the following we use amoc to refer only to the upper overturning cell owing to the large heat capacity of water the northwardflowing near surface waters contribute effectively to the heat transport from the tropics to the mid and high latitudes and therefore affect the regional climate around the north atlantic 9 10 hence changes of the amoc strength can substantially influence climate in the north especially in europe the depth of the southward flowing waters and the strength of the circulation have changed in the past during the last glacial period abrupt warming events of several degrees within a few decades followed by slower cooling were detected in greenland ice cores 11 as effects of the so called dansgaardoeschger events were found far beyond the greenland ice sheet 1214 with patterns reminiscent of varying inter hemispheric heat distribution 15 16 they are believed to be the result of rapid changes of the amoc strength 17 other examples of a reduction or even a complete shutdown of the

chronology of glacial lake agassiz meltwater routed to the gulf of mexico abstract sediment cores with new radiocarbon dates from the southern outlet of glacial lake agassiz indicate that meltwater delivery to the mississippi valley was disrupted at 10 800 14 c yr b p and the outlet was abandoned by 9400 14 c yr b p these findings confirm the timing of generally accepted terminations of the lockhart and emerson phases of lake agassiz additionally the radiocarbon chronology indicates that the spillway was fully formed by 10 800 14 c yr b p and that the occupancy in late emerson time was likely short lived with minimal spillway erosion

glacial lake agassiz its northwest maximum extent and outlet in saskatchewan emerson phase abstract six different lines of evidence support the hypothesis that glacial lake agassiz expanded an additional 70 000 km2 over that previously mapped in northwestern saskatchewan and that the lake discharged out the northwestern clearwater outlet then through glacial lake mcconnell and mackenzie river to the arctic ocean elevations of formerly unmapped 1 strandlines and 2 glaciolacustrine sediments between the previously mapped northwest limit of lake agassiz and the clearwater lower athabasca spillway indicate that water extended 170 km farther northwest recently mapped strandlines at elevations up to 60 m above the previously mapped extent of lake agassiz can be traced along 3 isobases to the mouth of the spillway based upon 4 six radiocarbon dates recovered from spillway flood deposits in the athabasca river valley and its late pleistocene delta the 5 clearwater spillway was cut at 9 9 ka bp this date is synchronous with the initiation of the emerson phase 9 9 ka bp in the southern lake agassiz basin and 6 coincides with the position of the laurentide ice sheet at the cree lake moraine 10 ka bp along the northern margin of the lake following closure of the eastern outlets at the onset of the emerson phase lake agassiz transgressed toward the northwest into the deglaciated and isostatically depressed glacial foreland in the churchill river valley to an elevation of 490 m the pre flood elevation of the churchill mackenzie drainage divide at the head of the clearwater lower athabasca spillway the beaver river moraine an earthen drainage divide was breached resulting in lowering lake agassiz 52 m to a stable elevation at 438 m the lake discharged 21 000 km3 of water into the arctic ocean that raised global sea level by 6 cm

mechanisms of abrupt climate change of the last glacial period 1 more than a decade ago ice core records from greenland revealed that the last glacial period was characterized by abrupt climate changes that recurred on millennial time scales since their discovery there has been a large effort to determine whether these climate events were a global phenomenon or were just confined to the north atlantic region and also to reveal the mechanisms that were responsible for them in this paper we review the available paleoclimate observations of abrupt change during the last glacial period in order to place constraints on possible mechanisms three different mechanisms are then reviewed ocean thermohaline circulation sea ice feedbacks and tropical processes each mechanism is tested for its ability to explain the key features of the observations particularly with regard to the abruptness millennial recurrence and geographical extent of the observed changes it is found that each of these mechanisms has explanatory strengths and weaknesses and key areas in which progress could be made in improving the understanding of their long term behavior both from observational and modeling approaches are suggested finally it is proposed that a complete understanding of the mechanisms of abrupt change requires inclusion of processes at both low and high latitudes as well as the potential for feedbacks between them some suggestions for experimental approaches to test for such feedbacks with coupled climate models are given

routing of meltwater from the laurentide ice sheet during the younger dryas cold episode rooth1 proposed that the younger dryas cold episode which chilled the north atlantic region from 11 000 to 10 000 yr bp was initiated by a diversion of meltwater from the mississippi drainage to the st lawrence drainage system the link between these events is postulated to be a turnoff during the younger dryas cold episode of the north atlantic s conveyor belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the north atlantic region2 this turnoff is attributed to a reduction in surface water salinity and hence also in density of the waters in the region where north atlantic deep water nadw now forms here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from orca basin core en32 pc4 which reveal a significant reduction in meltwater flow through the mississippi river to the gulf of mexico from about 11 200 to 10 000 radiocarbon years ago this finding is consistent with the record for lake agassiz which indicates that the meltwater from the southwestern margin of the laurentide ice sheet was diverted to the northern atlantic ocean through the st lawrence valley during the interval from 11 000 to 10 000 years before present yr bp

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

the allerod younger dryas holocene sequence in the west central champlain sea eastern ontario a record of glacial oceanographic and climatic changes abstract the aragonite mineralogy and geochemistry of the mollusc faunas preserved at navan and bearbrook ontario serve as proxies of original seawater chemistry the composite section spanning 12 980 10 980 cal yr bp includes the younger dryas yd paleoclimatic oscillation oxygen isotopes demonstrate the onset of cooling with the yd event in addition to the lowering of marine values by the influx of isotopically light glacial meltwater from lake agassiz impact of cooling and dilution is reduced or eliminated with the start of the holocene when water temperatures and salinities for champlain sea cs seawater were 8 16 c and 27 34 ppt respectively overall oxygen isotope values deceased to 3 5 during the yd mainly due to freshening by glacial meltwater carbon isotopes confirm the rise in atmospheric co 2 concentration at the yd holocene transition marine strontium isotope values for the allerod yd earliest holocene range from 0 709151 16 210 cal yr bp to 0 709145 12 980 cal yr bp and 0 709142 10 950 cal yr bp the oceanographic changes recorded for the cs are in agreement with the evolutionary phases of lake agassiz and deglaciation dynamics of the laurentide ice sheet the volume and direction of meltwater discharge from lake agassiz alternated between the gulf of mexico during the allerod via the great lakes through the cs to the north atlantic during the yd and back to the gulf of mexico during the early holocene but with diminished impact

climate change a glacial test of timing meticulous reconstruction of the former extent of a glacier high in the mountains of new zealand will help in interpreting global scale climatic adjustments that occurred at the end of the last glaciation

synchronous records of pco2 and δ14c suggest rapid ocean derived pco2 fluctuations at the onset of younger dryas just before the onset of the younger dryas yd cold event several stomatal proxy based pco2 records have shown a sharp increase in atmospheric co2 concentration pco2 of between ca 50 and 100 ppm followed by a rapid decrease of similar or even larger magnitude here we compare one of these records a high resolution pco2 record from southern sweden with the intcal13 record of radiocarbon d 14 c the two records show broadly synchronous fluctuations at the yd onset specifically the intcal13 record documents decreasing d 14 c just before the yd onset when pco2 peaks consistent with a source of old co2 from the deep ocean we propose that this fluctuation occurred due to a major ocean flushing event the cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor we confirm that the earth system can produce such large multidecadal timescale fluctuations in pco2 through simulating an artificial ocean flushing event with the genie earth system model we suggest that sharp transitions of pco2 may have remained undetected so far in ice cores due to inter firn gas exchange and time averaging the stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change

late quaternary stage 2 and 3 meltwater and heinrich events northwest labrador sea two major meltwater events are documented in cores from the nw labrador sea one occurred ca 20 000 14c yr b p in association with deposition of a major detrital carbonate unit both prior to and after this event 18o values of near surface planktonic foraminifera were 4 5 indicating fully enriched glacial values a younger event ca 14 000 14 c yr b p is characterized by a dramatic change in 18o from 4 5 to 2 0 and coincided with the retreat of ice from the outer se baffin shelf possibly into hudson strait these meltwater events coincide with heinrich h layers 1 and 2 from north atlantic sediments the 14 000 14c yr b p meltwater event indicates that the eastern margin of the laurentide ice sheet also underwent rapid retreat at approximately the same time as other ice sheet margins around the ne north atlantic a third major detrital carbonate event at the base of hu87 033 009 possibly correlative with heinrich layer 3 occurred ca 33 960 675 14 c yr b p however this is older than the accepted date for h 3 of 27 000 14c yr b p and may be h 4

sediments sedimentation rates and environments southeast baffin shelf and northwest labrador sea 8 26 ka ten 14c dated cores are described from the labrador sea continental slope and hatton and resolution basins on the southeast baffin shelf based on sharply defined detrital carbonate layers in the labrador sea cores we propose that the laurentide ice sheet reached the shelf break at both 20 and 15 14c ka 24 3 and 18 2 sidereal ka and contributed significant sediment to the northwest labrador sea both times the flux was 720 kg m2 ka during these periods of maximum ice extent the laurentide ice sheet retreated from its 15 ka position at the shelf break to the inner shelf between 14 and 12 14c ka ice proximal conditions recognized by relatively light 18o on near surface planktonic foraminifera high detrital carbonate and benthic foraminiferal faunas dominated by elphidium excavatum forma clavata cassidulina reniforme prevailed until at least 10 14c ka the ice readvanced toward resolution basin and across part of hatton basin at 11 14c ka accelerator mass spectrometry dates on core tops i

reduced meltwater outflow from the laurentide ice margin during the younger dryas the cause of the younger dryas cold event which interrupted the last deglaciation is still a matter of debate1 a prevalent hypothesis proposed by broecker et al 2 is that the abrupt climate change was driven by a decrease in the rate of north atlantic deep water production triggered by a sudden dilution of north atlantic surface water in response to the diversion of laurentide icesheet melt water from the mississippi drainage system to that of the st lawrence river here we investigate the feasibility of this triggering mechanism by reconstructing sea surface temperature salinity and sea ice cover records for the outlet of the gulf of st lawrence into the north atlantic ocean these reconstructions based on dinoflagellate cyst assemblages3 4 in sediment cores from the region show reduced meltwater runoff low temperatures and extensive sea ice cover during the younger dryas dated here between 10 800 and 10 300 bp meltwater pulses did occur before and after the younger dryas event as early as 11 700 bp during the development of the champlain sea in the st lawrence lowland and afterwards until 10 100 bp at the resolution of our salinity proxy 0 7 the meltwater pulses preceding the younger dryas did not affect sea surface salinity off the shelf break these constraint on the meltwater outflow through the st lawrence drainage system do not support the triggering mechanism of the broecker et al hypothesis2 unless the north atlantic thermohaline circulation is more sensitive to small salinity changes than most models suggest23

reply to comments by carlson et al 2009 on radiocarbon deglaciation chronology of the thunder bay ontario area and implications for ice sheet retreat patterns

importance of freshwater injections into the arctic ocean in triggering the younger dryas cooling the cause of past climate change has been the focus of many studies in recent years various explanations have been advanced to explain the record of quaternary climate change identified in sediments on continents in oceans and in the ice caps of greenland and antarctica important in this is the scale of change both temporal and spatial and our best records especially in terms of chronological resolution come from the past several hundred thousand years a number of climate fluctuations some of them abrupt and some of them short have been linked to changes in the flux of freshwater to the oceans that if large enough might have impacted on ocean circulation and in turn have resulted in a change in climate broecker et al 1 were among the first to propose that an injection of freshwater from north america was responsible for the anomalous younger dryas yd cooling 12 9 to 11 5 ka known from europe and other circum atlantic continental regions in their hypothesis an outburst of water from glacial lake agassiz once the largest lake in the world eastward through the great lakes and st lawrence valley to the north atlantic which coincided in time with the yd was identified as the mechanism for slowing thermohaline circulation thc and in turn for triggering the yd cooling subsequent field research led others to conclude that changes in overflow routing from this giant lake were responsible for the preboreal oscillation and 8 2 ka event 2 5 coupled ocean atmosphere modeling seemed to confirm these interpretations 6

a heinrich like event h 0 dc 0 source s for detrital carbonate in the north atlantic during the younger dryas chronozone in the north atlantic we define h 0 as a heinrich like event which occurred during the younger dryas chron on the se baffin shelf prior to 11 ka surface water productivity was reasonably high as measured by the numbers of diatom and planktic foraminifera per gram but an abrupt increase in detrital carbonate dc 0 event from approximately 15 up to 50 carbonate by weight occurred at 11 14c ka and continued to circa 10 ka these deposits 2 6 m thick are dominated by detrital calcite and silt and clay sized sediments during this event dc 0 h 0 ice extended onto the inner shelf but did not reach the shelf break and probably originated from a center over labrador ungava as a consequence the pattern of ice rafted debris and sediment provenance shown by h o in the north atlantic is different from that during h 1 14 5 ka or h 2 20 ka when the ice sheet extended along the axis of hudson strait and may have reached the shelf break for example there is no concrete evidence for dc o is cores on the floor of the labrador sea due east of hudson strait hu75 55 56 but h o has been noted in cores off newfoundland and west of ireland a coeval carbonate event to dc 0 but this one dominated by dolomite occurs in hu82 su5 on the west side of davis strait with a source either from northern baffin bay or cumberland sound although other sources for north atlantic detrital carbonate cannot be totally excluded our evidence suggests that h 0 represents the expression of glaciological instability of the laurentide ice sheet within the general region of hudson strait and probably to the north cumberland sound and northernmost baffin bay there is one younger dc event dated circa 8 4 ka present in sediments along the labrador margin and in hudson strait which represents the final collapse of the ice sheet within hudson strait and hudson bay

holocene climatic variations in the western cordillera of colombia a multiproxy high resolution record unravels the dual influence of enso and itcz abstract the paramo de frontino 3460 m elevation in colombia is located approximately halfway between the pacific and atlantic oceans it contains a 17 kyr long stratigraphically continuous sedimentary sequence dated by 30 ams 14c ages our study covers the last 11 500 cal yr and focuses on the biotic pollen and abiotic microfluorescence x or xrf components of this high mountain ecosystem the pollen record provides a proxy for temperature and humidity with a resolution of 20 35 yr and xrf of ti and fe is a proxy for rainfall with a sub annual ca 6 month resolution temperature and humidity display rapid and significant changes over the holocene the rapid transition from a cold mean annual temperature mat 3 5 c lower than today and wet younger dryas to a warm and dry early holocene is dated at 11 410 cal yr bp during the holocene mat varied from ca 2 5 c below to 3 5 above present day temperature warm periods 11 410 10 700 9700 6900 4000 2400 cal yr bp were separated by colder intervals the last 2 4 kyr of the record is affected by human impact the holocene remained dry until 7500 cal yr bp then precipitations increased to reach a maximum between 5000 and 4500 cal yr bp a rapid decrease occurred until 3500 cal yr bp and the late holocene was dry spectral analysis of xrf data show rainfall cyclicity at millennial scale throughout the holocene and at centennial down to enso scale in more specific time intervals the highest rainfall intervals correlate with the highest activity of enso variability in solar output is possibly the main cause for this millennial to decadal cyclicity we interpret enso and itcz as the main climate change driving mechanisms in frontino comparison with high resolution xrf data from the caribbean cariaco basin a proxy for rainfall in the coastal venezuelian cordilleras demonstrates that climate in frontino was pacific driven enso dominated during the yd and early holocene whereas it was atlantic driven in cariaco itcz dominated from ca 8000 cal yr bp climate in both areas was under the dual influence of enso and itcz thereby showing existing teleconnections between the tropical pacific and atlantic oceans the frontino record is to date the highest resolution holocene study in nw colombia an implication of these results is that new records should be analyzed with multiproxy tools in particular those providing high resolution time series such as xrf

impact of floods versus routing events on the thermohaline circulation 1 the last deglaciation was interrupted by three major cooling events the younger dryas the preboreal oscillation and the 8 2 ka cold event as the laurentide ice sheet retreated different outlets from lake agassiz became available causing the proglacial lake to drain to a new level in a rather short timescale flood and divert continental runoff over hundreds of years routing here we use an earth system model to investigate the impact of floods short term high amplitude versus routing events long term low amplitude on the thermohaline circulation and the resulting climate change we show that the initial outbursts of freshwater are not strong enough to influence large scale climate and can therefore not be the cause of millennial scale climate variability however routing events have the potential of weakening the thermohaline circulation and therefore causing a cool event in the northern hemisphere

glacial chronology and palaeoclimate in the bystra catchment western tatra mountains poland during the late pleistocene abstract deglaciation chronology of the bystra catchment western tatra mountains has been reconstructed based on 10be exposure age dating fourteen rock samples were collected from boulders located on three moraines that limit the horizontal extent of the lgm maximum advance and the lateglacial recessional stage the oldest preserved maximum moraine was dated at 15 5 0 8 ka an age that could be explained more likely by post depositional erosion of the moraine such scenario is supported by geomorphologic and palaeoclimatological evidence the younger cold stage is represented by well preserved termino lateral moraine systems in the kondratowa and sucha kasprowa valleys the distribution of the moraine ridges in both valleys suggest a complex history of deglaciation of the area the first late glacial re advance lg1 was followed by a cold oscillation lg2 that occurred at around 14 0 0 7 13 7 1 2 ka glaciers during both stages had nearly the same horizontal extent however their thickness and geometry changed significantly mainly due to local climatic conditions triggered by topography controlling the exposition to solar radiation the lg1 stage occurred probably during the pre bolling cold stage greenland stadial 2 1a however the lg2 stage can be correlated with the cooling at around 14 ka during the greenland interstadial 1 gi 1d older dryas this is the first chronological evidence of the older dryas in the tatra mountains the ela of the maximum bystra glacier was located at 1480 m a s l in accordance with the ela in the high tatra mountains during the lgm during the lg1 and lg2 stages the ela in the catchment rose up to 1520 1530 m a s l and was located approximately 100 150 m lower than in the eastern part of the massif climate modelling results show that the bystra glacier maximum advance could have advanced in the catchment when mean annual temperature was lower than today by 11 12 c and precipitation was reduced by 40 60 this is in accordance with lgm conditions previously reported for the high tatras during the lg1 and lg2 stages the temperature decrease in the study area reached 10 c and precipitation was lower by 30 compare to modern conditions this resulted in slightly higher accumulation 20 30 in the western tatra mountains compare to the high tatra mountains

catastrophic flooding origin of shelf valley systems in the english channel britain became geographically isolated from continental europe when high interglacial seas flooded the shallow english channel and north sea shelf areas but a tenuous link remained the weald artois chalk ridge between southeast england and northwest france just how this isthmus was breached making britain an island has been a matter of conjecture a new bathymetric map of the sea floor may solve the mystery it reveals a large bedrock floored valley containing landforms including grooves and streamlined islands consistent with a megaflood event caused when a rock dam at the dover strait breached draining a large pro glacial lake in the north sea basin a hypothesis for the permanent isolation of britain from mainland europe during interglacial high sea levels has been confirmed gupta et al analysed a new regional bathymetric map of part of the area finding a valley with landforms indicating large scale subaerial erosion by high magnitude water discharges they suggest breaching of a rock dam at the dover strait instigated drainage of a large lake in the north sea basin megaflood events involving sudden discharges of exceptionally large volumes of water are rare but can significantly affect landscape evolution continental scale drainage patterns and climate change1 it has been proposed that a significant flood event eroded a network of large ancient valleys on the floor of the english channel the narrow seaway between england and france2 3 4 this hypothesis has remained untested through lack of direct evidence and alternative non catastrophist ideas have been entertained for valley formation5 6 here we analyse a new regional bathymetric map of part of the english channel derived from high resolution sonar data which shows the morphology of the valley in unprecedented detail we observe a large bedrock floored valley that contains a distinct assemblage of landforms including streamlined islands and longitudinal erosional grooves which are indicative of large scale subaerial erosion by high magnitude water discharges our observations support the megaflood model in which breaching of a rock dam at the dover strait instigated catastrophic drainage of a large pro glacial lake in the southern north sea basin2 we suggest that megaflooding provides an explanation for the permanent isolation of britain from mainland europe during interglacial high sea level stands7 and consequently for patterns of early human colonisation of britain together with the large scale reorganization of palaeodrainage in northwest europe4

comparison of rockfall susceptibility assessment at local and regional scale a case study in the north of beijing china this study shows a rockfall susceptibility assessment at local scale in north beijing of china including the identification of rockfall sources onsite by terrain and rock discontinuities analysis and run out distance prediction by rocfall simulation two types of rockfall were defined including one type on the cliffs with long inclined slopes and another type on the road slopes with low height two historical rockfall events were used to back calibrating the parameters used for run out distance simulation based on the work rockfall susceptibility map at local scale was created in gis which was compared with the map obtained at regional scale entire huairou district scale due to the difference of approaches applied procedure of assessment and types of source data acquired the two resulting rockfall susceptibility maps are proved to be different still both of them are useful and could be used at different level s decision for rockfall prevention and mitigation different types of uncertainties exist in the study of rockfall susceptibility assessment to reduce the uncertainties studies on both approaches and techniques are suggested

the younger dryas black mat from ojo de agua a geoarchaeological site in northeastern zacatecas mexico abstract new explorations in the desert of northeastern zacatecas in central northern mexico revealed dozens of archaeological and geoarchaeological sites one of them ojo de agua contains the remains of a pleistocene spring fed hydrographic system located at the southeastern end of a large elongated endorheic basin the locality yielded a particularly dark highly organic stratigraphic layer commonly known in the americas as black mat bm exposed on the natural profiles of a creek but not associated with cultural materials several radiocarbon assessments confirmed the formation of the ojo de agua black mat during the younger dryas chronozone with ten calibrated results clustering between 12 700 12 100 cal bp this multi proxy study confirmed the peculiarity of the deposit and found similarities and differences with other contexts of younger dryas age the ojo de agua black mat stratum c2 is far richer in charcoal specks than the related strata but lacks phytoliths diatoms or ostracods no further biological remains were found in it except for intrusive capillary roots clearly water lain in a shallow pond the stratum qualifies as a clayey silt with an acidic to neutral ph rich in heavy metals and with high contents of titanium the ojo de agua black mat yielded significant indicators of intense wildfires during the younger dryas but produced no carbon spherules or nanodiamonds supposedly linked to the impact theory

deglacial δ18o and hydrologic variability in the tropical pacific and indian oceans abstract evidence from geologic archives suggests that there were large changes in the tropical hydrologic cycle associated with the two prominent northern hemisphere deglacial cooling events heinrich stadial 1 hs1 19 to 15 kyr bp kyr bp 1000 yr before present and the younger dryas 12 9 to 11 7 kyr bp these hydrologic shifts have been alternatively attributed to high and low latitude origin here we present a new record of hydrologic variability based on planktic foraminifera derived 18o of seawater 18osw estimates from a sediment core from the tropical eastern indian ocean and using 12 additional 18osw records construct a single record of the dominant mode of tropical eastern equatorial pacific and indo pacific warm pool ipwp hydrologic variability we show that deglacial hydrologic shifts parallel variations in the reconstructed interhemispheric temperature gradient suggesting a strong response to variations in the atlantic meridional overturning circulation and the attendant heat redistribution a transient model simulation of the last deglaciation suggests that hydrologic changes including a southward shift in the intertropical convergence zone itcz which likely occurred during these northern hemisphere cold events coupled with oceanic advection and mixing resulted in increased salinity in the indonesian region of the ipwp and the eastern tropical pacific which is recorded by the 18osw proxy based on our observations and modeling results we suggest the interhemispheric temperature gradient directly controls the tropical hydrologic cycle on these time scales which in turn mediates poleward atmospheric heat transport

mountain glacier evolution in the iberian peninsula during the younger dryas abstract we review the evolution of glaciers in the iberian mountains during the younger dryas 12 9 11 7 ka following the chronology of greenland stadial 1 gs1 and compare with available environmental and climate data to investigate glacier dynamics during cold stadial episodes the best examples of younger dryas moraines are found in the central pyrenees involving short ice tongues up to 4 km in length in the highest massifs above 3000 m a s l of the southern versant small cirque glaciers and rock glaciers formed during the yd occurred in other pyrenean massifs in the cantabrian range and in the gredos and guadarrama sierras central range as indicated by several rocky polished thresholds that were ice free at the beginning of the holocene although some former rock glaciers were re activated during the younger dryas glacial activity was limited in the southernmost part of the iberian peninsula sierra nevada most iberian records show vegetation changes during the yd characterized by a forest decline and an expansion of shrubs mainly juniperus and steppe herbs although the vegetation response was not homogeneous because of variable resilience among ecosystems available records also document a variable lake response in terms of hydrology and productivity with a decrease in sedimentation rates and organic productivity in most high altitude lakes and increases in salinity and relatively lower lake levels at lower altitudes the impact of the younger dryas on the coastal environment was almost negligible but it was responsible for a brief cessation in sea level rise high resolution analyses of new speleothem records have documented a double structure for the yd with an earlier drier phase followed by a relatively more humid period the review of geomorphological evidence demonstrates a strong latitudinal control of glacial activity during the yd with more intense development in the northern than in the southern regions the increase in humidity during the second phase of the yd in southern latitudes could have been a decisive factor for glacier advance in the northern iberian mountains

a multi proxy reconstruction of oceanographic conditions around the younger dryas holocene transition in placentia bay newfoundland abstract a marine sediment core from placentia bay off the south coast of newfoundland was studied to reconstruct past oceanographic conditions based on a multiproxy approach including diatoms benthic foraminifera grain size analysis ip25 x ray fluorescence calcium carbonate and total organic carbon content based on 6 radiocarbon dates the 513 5 cm long core spans the age interval from ca 13 to 10 cal kyrs bp and thus captures the transition from the younger dryas stadial into the holocene at sub decadal resolution the younger dryas was characterized by cold arctic water taxa of diatoms and benthic foraminifera and a stratified water column with high sea ice cover and low productivity due to strong influence of the labrador current during the second half of the younger dryas after ca 12 2 cal kyrs bp the influence of the labrador current gradually decreased and sea ice conditions became more variable the transition towards the holocene is characterized by a sequence of events starting with turnovers in the biogenic proxies followed by an abrupt retreat of sea ice and a rise in productivity these events are believed to be related to a northward migration of the gulf stream labrador current oceanic front the younger dryas termination is directly followed by a detrital carbonate layer associated with the final phase of the heinrich 0 event linked to laurentide ice sheet dynamics during the early holocene productivity remained high with minimal sea ice cover reduced stratification and stronger influence of warmer gulf stream derived waters

glaciation of scotland during the younger dryas stadial a review considerable research effort in recent years has refined our knowledge of the landforms and sedimentary sequences generated by glaciers of the last cold episode in scotland the younger dryas and the chronology of their formation these recent advances have benefited from new technologies such as high resolution digital terrain models numerical ice sheet simulations and cosmic ray surface exposure dating this review presents a summary of recent younger dryas research in scotland particularly focusing on studies from the last two decades the findings of these investigations are compared with those of earlier workers and critically are appraised in terms of their relevance to five key aspects of younger dryas glaciation growth of the ice cap its geometry dynamics mode of decay and likely stadial climate minor differences aside the consensus view appears to favour a relatively thick but areally restricted central ice cap surrounded by satellite ice fields at its centre the ice cap was probably thickest during the first 500 years of the stadial after which it most likely thinned as the climate became increasingly arid outlet glaciers were dynamic only in marginal areas where they produced abundant moraine sequences during deglaciation but throughout much of the stadial central areas of the ice cap effected only minimal landscape modification deglaciation took place by active retreat under a rapidly warming climate overall the stadial was dominated by much colder and drier conditions than present and probably experi enced greater intra annual temperature ranges copyright 2009 john wiley sons ltd