detrital carbonate peaks on the labrador shelf a 13 7 ka template for freshwater forcing from the hudson strait outlet of the laurentide ice sheet into the subpolar gyre abstract the laurentide ice sheet lis was a large dynamic ice sheet in the early holocene the glacial events through hudson strait leading to its eventual demise are recorded in the well dated labrador shelf core md99 2236 from the cartwright saddle we develop a detailed history of the timing of ice sheet discharge events from the hudson strait outlet of the lis during the holocene using high resolution detrital carbonate ice rafted detritus ird 18o and sediment color data eight detrital carbonate peaks dcps associated with ird peaks and light oxygen isotope events punctuate the md99 2236 record between 11 5 and 8 0 ka we use the stratigraphy of the dcps developed from md99 2236 to select the appropriate r to calibrate the ages of recorded glacial events in hudson bay and hudson strait such that they match the dcps in md99 2236 we associate the eight dcps with h0 gold cove advance noble inlet advance initial retreat of the hudson strait ice stream hsis from hudson strait opening of the tyrrell sea and drainage of glacial lakes agassiz and ojibway the opening of foxe channel and retreat of glacial ice from foxe basin are represented by a shoulder in the carbonate data r of 350 years applied to the radiocarbon ages constraining glacial events h0 through the opening of the tyrell sea provided the best match with the md99 2236 dcps r values and ages from the literature are used for the younger events a very close age match was achieved between the 8 2 ka cold event in the greenland ice cores dcp7 8 15 ka bp and the drainage of glacial lakes agassiz and ojibway our stratigraphic comparison between the dcps in md99 2236 and the calibrated ages of hudson strait bay deglacial events shows that the retreat of the hsis the opening of the tyrell sea and the catastrophic drainage of glacial lakes agassiz and ojibway at 8 2 ka are separate events that have been combined in previous estimates of the timing of the 8 2 ka event from marine records sw iceland shelf core md99 2256 documents freshwater entrainment into the subpolar gyre from the hudson strait outlet via the labrador north atlantic and irminger currents the timing of freshwater release from the lis hudson strait outlet in md99 2236 matches evidence for freshwater forcing and lis icebergs carrying foreign minerals to the sw iceland shelf between 11 5 and 8 2 ka the congruency of these records supports the conclusion of the entrainment of freshwater from the retreat of the lis through hudson strait into the subpolar gyre and provides specific time periods when pulses of lis freshwater were present to influence climate

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

inferring surface water equilibrium calcite δ18o during the last deglacial period from benthic foraminiferal records implications for ocean circulation the ocean circulation modifies mixed layer ml tracer signals as they are communicated to the deep ocean by advection and mixing we develop and apply a procedure for using tracer signals observed upstream by planktonic foraminifera and downstream by benthic foraminifera to constrain how tracer signals are modified by the intervening circulation and by extension to constrain properties of that circulation a history of ml equilibrium calcite 18o 18oc spanning the last deglaciation is inferred from a least squares fit of eight benthic foraminiferal 18oc records to green s function estimated for the modern ocean circulation disagreements between this history and the ml history implied by planktonic records would indicate deglacial circulation changes no deviations from the modern circulation are diagnosed because the two estimates of ml 18oc agree within their uncertainties over the deglaciation but diagnostics suggest data collection and modeling procedures useful for inferring circulation changes uncertainties of benthic derived ml 18oc are lowest in the high latitude regions chiefly responsible for ventilating the deep ocean additional high resolution planktonic records constraining these regions are of particular utility benthic records from the southern ocean where data are sparse appear to have the most power to reduce uncertainties in benthic derived ml 18oc understanding the spatiotemporal covariance of deglacial ml 18oc will also improve abilities of 18oc records to constrain deglacial circulation

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

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

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

atlantic pacific seesaw and its role in outgassing co2 during heinrich events 1 paleoproxy records indicate that a marked weakening of the atlantic meridional overturning circulation amoc during heinrich events was often accompanied by a notable atmospheric co2 increase however previous modeling studies display conflicting atmospheric co2 responses to an amoc shutdown here we use model simulations combined with paleoproxy records to show that depending on the deep and bottom water transport in the northern and southern pacific ocean during an amoc weakening the ocean can act either as a sink or a source of carbon results from idealized meltwater experiments as well as from a transient experiment covering heinrich stadial 4 suggest that a shutdown of the amoc during heinrich stadials 4 hs4 and 1 hs1 led to an enhancement of antarctic bottom water aabw and north pacific deep water npdw transport we show that enhanced deep and bottom water transport in the pacific ocean ventilates deep pacific carbon through the southern ocean thus contributing to a rise in atmospheric co2 this mechanism yields a good agreement between paleoproxy records and modeling results thus highlighting the possible establishment of an atlantic pacific seesaw during heinrich stadials enhanced aabw and npdw transport could account for most of the observed atmospheric co2 increase during hs4 and for about 30 of the atmospheric co2 increase during hs1

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

hydroacoustic detection of volcanic activity on the gorda ridge february march 1996 abstract beginning at 0700 gmt on 28 february 1996 intense seismicity was detected in the northeast pacific ocean using the t phase monitoring system developed by noaa pmel to access the u s navy s sound surveillance system sosus in the north pacific the event was preliminarily located on the northernmost segment of the gorda ridge near 42 67 n and 126 8 w in the vicinity of the ridge segment high narrowgate the nature of the seismicity was similar to that observed in june 1993 at the coaxial segment of the juan de fuca ridge which was later documented to be a lateral magma injection with subsequent eruption due to several gaps in the data the detection information was not as comprehensive as during the coaxial event but an initial migration of epicenters from the narrowgate area down rift is inferred based on arrival bearings from a single array there is evidence for an additional diking event on the second and third day of activity there is also indication of a concentration of epicenters located near 42 6 n as occurred during the coaxial episode at what was later determined to be an eruption site examination of t wave rise times generally supports this interpretation based on the nature and duration of the activity a response effort was initiated which later confirmed hot water plumes and fresh lava flows at the site based on both hydroacoustic information and field observations it is proposed that the episode began with a lateral dike injection possibly with eruptive activity in the summit region followed by multiple magma pulses and eventual focusing of the seismic activity and extrusion near 42 6 n

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

a subtropical fate awaited freshwater discharged from glacial lake agassiz 1 the 8 2 kyr event is the largest abrupt climatic change recorded in the last 10 000 years and is widely hypothesized to have been triggered by the release of thousands of kilometers cubed of freshwater into the north atlantic ocean using a high resolution 1 6 global ocean ice circulation model we present an alternative view that freshwater discharged from glacial lake agassiz would have remained on the continental shelf as a narrow buoyant coastal current and would have been transported south into the subtropical north atlantic the pathway we describe is in contrast to the conceptual idea that freshwater from this lake outburst spread over most of the sub polar north atlantic and covered the deep open ocean convection regions this coastally confined freshwater pathway is consistent with the present day routing of freshwater from hudson bay as well as paleoceanographic evidence of this event using a coarse resolution 2 6 version of the same model we demonstrate that the previously reported spreading of freshwater across the sub polar north atlantic results from the inability of numerical models of this resolution to accurately resolve narrow coastal flows producing instead a diffuse circulation that advects freshwater away from the boundaries to understand the climatic impact of freshwater released in the past or future e g greenland and antarctica the ocean needs to be modeled at a resolution sufficient to resolve the dynamics of narrow coastal buoyant flows

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

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

buoyant gravity currents along a sloping bottom in a rotating fluid author posting cambridge university press 2002 this article is posted here by permission of cambridge university press for personal use not for redistribution the definitive version was published in journal of fluid mechanics 464 2002 251 278 doi 10 1017 s0022112002008868

meltwater routing and the younger dryas the younger dryas the last major cold episode on earth is generally considered to have been triggered by a meltwater flood into the north atlantic the prevailing hypothesis proposed by broecker et al 1989 nature 341 318 321 more than two decades ago suggests that an abrupt rerouting of lake agassiz overflow through the great lakes and st lawrence valley inhibited deep water formation in the subpolar north atlantic and weakened the strength of the atlantic meridional overturning circulation amoc more recently tarasov and peltier 2005 nature 435 662 665 showed that meltwater could have discharged into the arctic ocean via the mackenzie valley 4 000 km northwest of the st lawrence outlet here we use a sophisticated high resolution ocean sea ice model to study the delivery of meltwater from the two drainage outlets to the deep water formation regions in the north atlantic unlike the hypothesis of broecker et al freshwater from the st lawrence valley advects into the subtropical gyre 3 000 km south of the north atlantic deep water formation regions and weakens the amoc by 30 we conclude that meltwater discharge from the arctic rather than the st lawrence valley was more likely to have triggered the younger dryas cooling

rock physics relationships between inverted elastic reflectivities quantitative relationships between true elastic properties are not always applicable to inverted elastic properties we use the important example of two term avo reflectivity inversion to demonstrate this avo literature includes many examples of two term avo equations and we provide a general relationship to convert between inverted reflectivities obtained when using these various equations conversion between inverted reflectivities should be based on this equation rather than standard elastic property relationships this is of immediate importance when applying the method of chi angle projections to estimate elastic reflectivities

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

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

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

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

stable isotope evidence for glacial lake drainage through the st lawrence estuary eastern canada 13 1 12 9 ka abstract postglacial varved and rhythmically laminated clays deposited during the transition from glacial lake vermont lv to the champlain sea cs record hydrological changes in the champlain st lawrence valley cslv at the onset of the younger dryas 13 1 12 9 ka linked to glacial lake drainage events oxygen isotope 18o records of three species of benthic foraminifera cassidulina reniforme haynesina orbiculare islandiella helenae from six sediment cores and the freshwater ostracode candona from one core were studied results show six large isotope excursions 0 5 to 2 in c reniforme 18o values five excursions in h orbiculare

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

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

lake agassiz final drainage event in the northwest north atlantic 1 the 8 2 ka climate event recorded in greenland ice cores is subject of debates with respect to causal linkage with a collapse of the atlantic meridional overturning due to the drainage of the late glacial lake agassiz here we present records from the nw north atlantic down current the flood discharge route showing that the 9 5 8 ka interval was marked by a succession of events the drainage itself corresponds to a twin layer of carbonate rich turbidites deposited within the calibrated 8 35 8 5 ka interval proxies of sea surface and deep current conditions do not indicate significant concomitant changes in the nw north atlantic the dataset however supports the concept that the 8 2 ka climate event may represent one of the manifestations of climate instability during an interval with major changes of land drainage in ne america due to the collapse of the laurentide ice sheet subsequent fast sea level rise and large scale reorganization of the north atlantic thermohaline circulation pattern

tracer transport timescales and the observed atlantic pacific lag in the timing of the last termination 1 the midpoint of the last termination occurred 4 000 years earlier in the deep atlantic than the deep pacific according to a pair of benthic foraminiferal 18o records seemingly implying an internal circulation shift because the lag is much longer than the deep radiocarbon age here a scenario where the lag is instead caused by regional surface boundary condition changes delays due to oceanic transit timescales and the interplay between temperature and seawater 18o 18ow is quantified with a tracer transport model of the modern day ocean circulation using an inverse method with individual green functions for 2 806 surface sources a time history of surface temperature and 18owis reconstructed for the last 30 000 years that is consistent with the foraminiferal oxygen isotope data mg ca derived deep temperature and glacial pore water records thus in the case that the ocean circulation was relatively unchanged between glacial and modern times the interbasin lag could be explained by the relatively late local glacial maximum around antarctica where surface 18ow continues to rise even after the north atlantic 18owfalls the arrival of the signal of the termination is delayed at the pacific core site due to the destructive interference of the still rising antarctic signal and the falling north atlantic signal this scenario is only possible because the ocean is not a single conveyor belt where all waters at the pacific core site previously passed the atlantic core site but instead the pacific core site is bathed more prominently by waters with a direct antarctic source

constraining the variability of the atlantic meridional overturning circulation during the holocene there is a converging body of evidence supporting a measurable slowdown of the atlantic meridional overturning circulation amoc as climate warms and northern hemisphere ice sheets inexorably shrink within this context we assess the variability of the amoc during the holocene based on a marine sediment core retrieved from the deep northwest atlantic which sensitively recorded large scale deglacial transitions in deep water circulation while there is a diffuse notion of holocene variability in labrador and nordic seas overturning we report a largely invariable deep water circulation for the last 11 000 years even during the meltwater pulse associated with the 8 2 ka event sensitivity tests along with high resolution 231pa 230th data constrain the duration and the magnitude of possible holocene amoc variations the generally constant baseline during the holocene suggests attenuated natural variability of the large scale amoc on submillennial timescales and calls for compensating effects involving the upstream components of north atlantic deep water