return of the coral reef hypothesis basin to shelf partitioning of caco3 and its effect on atmospheric co2 differences in the rate of coral reef carbonate deposition from the pleistocene to the holocene may account for the quaternary variation of atmospheric co sub 2 volumes of carbonate associated with holocene reefs require an average deposition rate of 2 0 times 10 sup 13 mol yr for the past 5 ka in light of combined riverine mid ocean ridge and ground water fluxes of calcium to the oceans of 2 3 times 10 sub 13 mol yr the current flux of calcium carbonate to pelagic sediments must be far below the pleistocene average of 1 2 times 10 sub 13 mol yr the authors suggest that sea level change shifts the locus of carbonate deposition from the deep sea to the shelves as the normal glacial interglacial pattern of deposition of quaternary global carbonates to assess the impact of these changes on atmospheric co sub 2 a simple numerical simulation of the global carbon cycle was developed atmospheric co sub 2 as well as calcite saturation depth and sediment responses to these carbonate deposition changes are examined atmospheric co sub 2 changes close to those observed in the vostok ice core approximately 80 ppm co sub 2 for the quaternary are observed as well as the approximate depth changes in percent carbonate of sediments measured in themore pacific ocean over the same time interval 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

application of an inverse method to interpret 231pa 230th observations from marine sediments constraints then sediment 231 pa 230 th data for each time interval are combined with an advection scavenging model in order to determine their in consistency with the modern circulation estimate we find that the majority of sediment 231 pa 230 th data for the holocene lgm or h1 can be brought into consistency with the modern circulation if plausible assumptions are made about the large scale distribution of 231 pa and about model uncertainties moreover the adjustments in the data needed to reach compatibility with a hypothetical state of no flow no advection are positively biased for each time interval suggesting that the 231 pa 230 th data including that for h1 are more consistent with a persistence of some circulation than with no circulation our study does not imply that earlier claims of a circulation change during the lgm or h1 are inaccurate but that these claims cannot be given a rigorous basis given the current uncertainties involved in the analysis of the 231 pa 230 th data

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

abrupt changes in antarctic intermediate water circulation over the past 25 000 years the formation and circulation of antarctic intermediate water has varied over glacial interglacial timescales a neodymium record from the atlantic ocean basin suggests that changes in circulation may have been driven by changes both in antarctic intermediate water formation in the southern ocean and in the strength of north atlantic meridional overturning

231pa 230th fractionation by ocean transport biogenic particle flux and particle type 231pa and 230th are removed from the water column by a process of reversible scavenging which quickly removes 230th to the sediment 231pa is less efficiently scavenged onto particles than 230th and is therefore more effectively transported via advection and diffusion before it reaches the ocean sediment this study combines particle fields dust opal caco3 poc derived from observations with the bern3d intermediate complexity ocean model and an equilibrium scavenging model for isotopes the equilibrium partition coefficient for particulate versus dissolved isotope activity is varied with particle type the model can explain many of the features of the global 231pa and 230th distribution the success of such a simple model at representing the global pattern of 231pa 230th activity ratio supports the use of this proxy in paleoceanographic studies we use the model to address the controversy concerning which particle types are dominant in fractionating 231pa 230th in the ocean the lithogenic dust flux is found to be unimportant for 231pa 230th fractionation the ocean fractionation of 231pa 230th is dominated by the distribution of the caco3 and opal flux we also confirm that opal is a weak scavenger of 230th

centennial scale variability of the british ice sheet implications for climate forcing and atlantic meridional overturning circulation during the last deglaciation evidence from paleoclimatic archives suggests that earth s climate experienced rapid temperature changes associated with pronounced interhemispheric asymmetry during the last glacial period explanations for these climate excursions have converged on nonlinear interactions between ice sheets and the ocean s thermohaline circulation but the driving mechanism remains to be identified here we use multidecadal marine records of faunal oxygen isotope and sediment proxies from the northeast atlantic proximal to the western margins of the last glacial british ice sheet bis to document the coupling between ice sheet dynamics ocean circulation and insolation changes the core data reveal successions of short lived 80 100 years high amplitude ice rafted debris ird events that were initiated up to 2000 years before the deposition of detrital carbonate during heinrich events he 1 and 2 progressive disintegration of the bis 19 16 kyr before present b p occurred in response to abrupt ocean climate warmings that impinged on the northeast atlantic during the early deglaciation peak ird deposition recurs at 180 220 year intervals plausibly involving repeated breakup of glacial tidewater margins and fringing marine ice shelves the early deglaciation culminated in a major meltwater pulse at 16 3 kyr b p followed by another discharge associated with he1 some 300 years after we conclude that temperature changes related to external forcing and marine heat transport caused a rapid response of the bis and possibly other margins of the eurasian ice sheet massive but short lived meltwater surges influenced the atlantic meridional overturning circulation thereby contributing to north atlantic climate variability and bipolar climatic asymmetry

evidence from the northeastern atlantic basin for variability in the rate of the meridional overturning circulation through the last deglaciation a first study from the subtropical western atlantic using 231pa 230th ratios as a kinematic proxy for deep water circulation provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation moc over the last deglaciation however these results warrant confirmation from additional locations and water depths because the interpretation of the sedimentary 231pa 230th ratio in terms of circulation vigor can be biased by variations in particle flux and composition we have measured 231pa 230th in a core from the iberian margin in the northeastern atlantic basin and have compared these new results to the data from the western atlantic basin we find that the reduction in the circulation during h1 and yd and the subsequent increases first recognized in the sediment deposited on bermuda rise are also evident in the eastern basin in a totally different sedimentary regime confirming that sedimentary 231pa 230th ratios record basin wide changes in deep water circulation however some differences between the eastern and western records are also recognized providing preliminary evidence to differentiate between renewal rates in the two north atlantic basins and between shallower and deeper overturning our results suggest the possible existence of two sources of glacial north atlantic intermediate deep water gnaiw one in the south labrador sea and another west of rockall plateau both sources contributed to the meridional overturning but the two had different sensitivity to meltwater from the laurentide and the fennoscandian ice sheets during the deglaciation these results indicate that additional information on the geometry and strength of the ventilation of the deep atlantic can be obtained by contrasting the evolution of sediment 231pa 230th in different sections of the atlantic ocean

variation in surface and deep water circulation in the denmark strait north atlantic during marine isotope stages 3 and 2 1 the paleoceanography in the denmark strait north atlantic is reconstructed based on down core analysis of planktonic foraminifera carbon and oxygen isotopes carbonate and ice rafted debris ird the stratigraphy spans 57 to 6 calibrated cal ka years and reveals sea surface temperatures that covary with greenland ice core temperatures throughout marine isotope stages mis 3 and 2 planktonic 18o minima interpreted as meltwater events follow sea surface warming ird from both the greenland and iceland ice sheets was delayed by 2 ka relative to the north atlantic heinrich events ventilation of bottom water was mostly sustained through open ocean convection associated with north atlantic deep water production or by convection of glacial north atlantic intermediate water except for two episodes at 54 cal ka mis 3 31 and 18 15 cal ka termination i during these two periods we suggest that deep water ventilation was accomplished mainly by brine formation

on the importance of opal carbonate and lithogenic clays in scavenging and fractionating 230th 231pa and 10be in the ocean abstract the influence of particle composition on the scavenging of 230 th 231 pa and 10 be in the ocean is critically re examined based on the particle solution distribution coefficient k d data reported by chase et al earth planet sci lett 204 2002 215 229 our re examination shows that the k d s for these radionuclides are strongly influenced by the amount of lithogenic material lithogenics in particulates as reported previously by us and that a strong correlation between the lithogenic and carbonate contents in open ocean particulates results in an apparent non linear correlation between caco 3 and k d for each of the three radionuclides using a three end member model we estimate that the respective k d s g g of 230 th 231 pa and 10 be in the open ocean are 2 3 10 8 1 2 10 7 and 3 8 10 6 for lithogenics 1 0 10 6 1 0 10 5 and 3 0 10 4 for carbonate and 2 5 10 5 1 2 10 6 and 7 5 10 5 for opal suggesting that lithogenics have a much stronger affinity for any of the three nuclides than does carbonate or opal lithogenics and carbonate preferentially scavenge 230 th over 231 pa and 10 be whereas opal slightly favors 231 pa and 10 be over 230 th relative to lithogenics carbonate plays a very limited role in fractionating 230 th 231 pa and 10 be due to its orders of magnitude smaller k d values for the three nuclides our results indicate that fractionations between pa and th and between be and th through particle scavenging are mainly determined by the opal to lithogenic ratio in particulates rather than the opal to carbonate ratio suggested by chase et al

deepwater circulation on blake outer ridge western north atlantic during the holocene younger dryas and last glacial maximum three depth transects containing a total of 33 sediment cores were investigated along the blake outer ridge in the western subtropical north atlantic sortable silt mean grain size and stable isotope records were used to assess the position and relative intensity of the western boundary undercurrent wbuc during the holocene the last glacial maximum lgm and the younger dryas yd intervals the holocene reconstruction is consistent with modern physical and chemical hydrographic measurements in the area suggesting a deep position for the fast flowing core of the wbuc 3000 4000 m deepening to 4500 m water depth on the ridge flanks and a water column dominated by north atlantic deep water nadw the lgm and yd reconstructions show that a comparable hydrographic regime was present during both these intervals suggesting a similar mode of circulation that was appreciably different from the holocene reconstruction the wbuc s zone of maximum flow speed during these intervals is suggested to have shifted above 2500 m water depth consistent with nutrient depleted glacial north atlantic intermediate water formation with an increasing influence of southern source water ssw beneath below 4000 m water depth results hint at increased ssw flow vigor during both the lgm and yd with higher flow speeds than during the holocene this study provides a framework for aiding the interpretation of time series records of paleocurrent flow speed changes in the region of the wbuc

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

qualitative and quantitative approaches in palaeohydrography a case study from core top parameters in the gulf of cadiz palaeoceanography relies on the assumption that parameters measureable in sediment cores correlate reliably to hydrographic parameters areas of dynamic watermass mixing which generally have steep hydrographic gradients therefore provide both a tempting target large spatial and temporal differences in temperature and salinity and a significant challenge high flow velocities and high noise for palaeoceanographers here we investigate the ability of parameters measured on core tops to be used as the basis for simulating regional hydrography within one globally important area of watermass mixing the gulf of cadiz using grainsize sediment composition benthic foraminiferal assemblage and benthic foraminiferal stable isotope analysis it is possible to build an effective framework for qualitative constraint of the position of the modern mediterranean outflow water mw plume we propose that the gulf of cadiz slope can be sub divided into 8 hydrographic zones reflecting these parameters proximal mw core mw1 steep slopes core mw1 gentle slopes distal mw1 distal mw2 lower limit of mw plume north atlantic deep water and ambient atlantic water we anticipate that compilation of time slice data will reveal hydrographic sub divisions of the slope for the past in a similar manner improving our understanding of past changes in the size and position of the mw plume but more work needs to be done before a secure protocol for quantitative reconstruction can be created stable isotope analysis alone is insufficient for the task with 13c behaving in a non conservative manner and 18ocalcite acting in an ambiguous way due to the competing influences of 18owater and temperature it is likely that if a single parameter that unambiguously determines the difference between mediterranean water and atlantic water can be identified combination with 18ocalcite will allow extension of our qualitative analysis into a quantitative means of palaeohydrographic reconstruction for this region similar complexity to that found in this study would be expected in any mixing region and in particular on the majority of sediment drifts we recommend that core top surveys similar to this study are performed on these sediment drift systems before any form of quantitative palaeohydrology is attempted

coupling of millennial scale changes in sea surface temperature and precipitation off northeastern brazil with high latitude climate shifts during the last glacial period 1 high resolution records of alkenone derived sea surface temperatures and elemental ti ca ratios from a sediment core retrieved off northeastern brazil 4 s reveal short term climate variability throughout the past 63 000 a large pulses of terrigenous sediment discharge caused by increased precipitation in the brazilian hinterland coincide with heinrich events and the younger dryas period terrigenous input maxima related to heinrich events h6 h2 are characterized by rapid cooling of surface water ranging between 0 5 and 2 c this signature is consistent with a climate model experiment where a reduction of the atlantic meridional overturning circulation amoc and related north atlantic cooling causes intensification of ne trade winds and a southward movement of the intertropical convergence zone resulting in enhanced precipitation off northeastern brazil during deglaciation the surface temperature evolution at the core site predominantly followed the antarctic warming trend including a cooling prior to the younger dryas period an abrupt temperature rise preceding the onset of the bolling allerod transition agrees with model experiments suggesting a southern hemisphere origin for the abrupt resumption of the amoc during deglaciation caused by southern ocean warming and associated with northward flow anomalies of the south atlantic western boundary current

similar rates of modern and last glacial ocean thermohaline circulation inferred from radiochemical data today the ocean thermohaline circulation transports half of the 231pa produced by radioactive decay in the atlantic ocean water column to the southern ocean this export respectively imparts low and high 231pa 230th ratios to the surface sediments of these oceans ocean sediments from the last glacial maximum bear a similar isotopic fingerprint implying that advection of north atlantic deep intermediate water into the circumpolar deep water of the southern ocean occurred at a similar or slightly higher rate during the last glacial period

changes in northeast atlantic hydrology during termination 1 insights from celtic margin s benthic foraminifera abstract using benthic foraminiferal based proxies in sediments from the celtic margin we provide a well dated record across the last deglaciation of the channel river dynamics and its potential impact on the hydrology of intermediate water masses along the european margin our results describe three main periods 1 during the last glacial maximum and before 21 ka bp the predominance of meso oligotrophic species suggests well oxygenated water masses after 21 ka bp increasing proportions of eutrophic species related to enhanced riverine supply occurs concomitantly with early warming in greenland air temperatures 2 a thick laminated deposit occurring during a 1500 years long period of seasonal melting of the european ice sheet eis is associated with early heinrich stadial 1 period 18 2 16 7 ka bp the benthic proxies describe low salinity episodes cold temperatures severe dysoxia and eutrophic conditions on the sea floor perhaps evidence for cascading of turbid meltwaters 3 during late hs1 16 7 14 7 ka bp conditions on the celtic margin s seafloor changed drastically and faunas indicate oligotrophic conditions as a result of the ceasing of eis meltwater discharges while surface waters were cold due to laurentide ice sheet lis icebergs releases increasing benthic mg ca ratios reveal a progressive warming of intermediate water masses whereas oxygen proxies indicate overall well oxygenated conditions in addition to the well known effect of eis meltwaters on surface waters in the celtic margin our benthic record documents a pronounced impact on intermediate water depths during hs1 which coincided with major amoc disruptions

ocean thermohaline circulation and sedimentary 231pa 230th ratio holocene sediments from the atlantic are characterized by 231pa 230th ratios below the production ratio of the two radionuclides in the water column 0 093 whereas holocene sediments from the southern ocean have 231pa 230th 0 093 this pattern of 231pa deficit and excess was ascribed to southward 231pa export from the atlantic by the atlantic thermohaline circulation thc as pa is scavenged less efficiently by marine particles and more effectively transported by the thc than th the same pattern is observed in deposits of the last glacial maximum lgm which led to a previous contention that the thc strength did not vary markedly through the last glacial termination here we embed a description of trace metal scavenging into a zonally averaged circulation biogeochemistry ocean model to explore the sensitivity of 231pa 230th in atlantic and southern ocean sediments to thc changes our results show that the production of biogenic opal which unlike other marine particles poorly fractionates th and pa in the southern ocean water column determines the spatial pattern of the sensitivity also 231pa 230th increases in the north atlantic but changes little in the south atlantic and decreases in the southern ocean as thc is reduced the mean 231pa 230th of the whole atlantic is therefore less sensitive to thc changes than the mean 231pa 230th of the north atlantic the current uncertainties in atlantic mean 231pa 230th are too large to rule out a twofold reduction of the thc at the lgm however the increase in north atlantic mean 231pa 230th simulated in response to a twofold thc reduction is larger than the observed change in the north atlantic mean 231pa 230th from the lgm to holocene comparing this change with the modeled sensitivity of north atlantic 231pa 230th to thc variations indicates that the thc at the lgm could not have been reduced by 30 of its present strength experiments of transient thc changes indicate that high resolution 231pa 230th records from north atlantic sediments could also document thermohaline oscillations on century to millennial timescales

upper ocean circulation in the glacial north atlantic from benthic foraminiferal isotope and trace element fingerprinting benthic cd ca and 13c records from the midlatitude and northern north atlantic are used to derive nutrient inventories and water mass distribution patterns for the past 50 000 years inferred holocene water column cd concentrations cdw and 13c values are 0 17 0 24 nmol kg 1 and 1 0 1 3 peedee belemnite pdb which document the dominance of nutrient depleted mediterranean outflow water mow and upper north atlantic deep water unadw glacial benthic cd ca and 13c indicate a continued contribution of unadw to the northern north atlantic and upper portuguese margin cdw 0 08 nmol kg 1 13c 1 86 pdb at the upper moroccan margin glacial cdw 0 23 nmol kg 1 is higher and 13c 1 44 pdb is lower during heinrich events benthic 13c decreases by up to 1 3 and peak cd ca increases by 0 1 0 14 mol mol 1 water column phosphorus equivalents are 1 8 2 8 mol kg 1 the combined cd ca and 13c pattern indicates that during mean glacial conditions antarctic intermediate water aaiw reached the midlatitude northeast atlantic 30 n during heinrich events aaiw contribution maximized so that southern hemisphere waters filled the north atlantic basin from bottom water to middepth levels

deep western boundary current variability in the subtropical northwest atlantic ocean during marine isotope stages 12 10 high resolution 125 500 year temporal resolution sortable silt mean grain size data of subtropical northwest atlantic ocean drilling program odp leg 172 site 1061 monitor fluctuations of the deep western boundary current dwbc throughout marine oxygen isotope stages mis 12 10 330 460 kyr in great detail comparison with foraminiferal stable isotope data from the same site and odp site 1063 bermuda rise data of poli et al 2000 shows that the dwbc shoals during glacial times in relation to a reduced north atlantic deep water production and increased advection of southern ocean waters superimposed fluctuations in the flow speed position of the dwbc at a semiprecession scale are persistent throughout mis 10 12 and independent of the glacial state additional millennial scale variations in dwbc flow speed parallel surface water cooling events and ice rafted debris episodes especially at the transition of mis 11 10 indicating the sensitivity of deep current flow variations to changes in the surface water conditions these millennial scale variations are amplified when benthic foraminiferal 18o values exceed 3 8 and may be related to the transition of an ice volume threshold where climate variability is possibly amplified by feedback mechanisms associated with ice sheet growth

were the north atlantic heinrich events triggered by the behavior of the european ice sheets theories explaining the origin and distribution of heinrich layers abrupt massive discharges of ice rafted detritus generally attribute these layers to the laurentide ice sheet however doubt has been cast by the discovery that sediments also originated from european ice sheets here we present isotopic sr nd fingerprints of heinrich layers obtained in cores from the european margin they reveal that for most of the heinrich layers the icebergs were calved first from the european ice sheets predating the laurentide surges accelerator mass spectrometry 14c ages reveal that the oceanic heinrich events were synchronous with their dust equivalents in the greenland ice cores implying a direct ocean atmosphere link the typical laurentide surge seems to have occurred over a short time span after about 1 5 k y of iceberg release from the european ice sheets this suggests that the laurentide ice sheet responded to a stimulus created by events in the european sector

enhanced arabian sea intermediate water flow during glacial north atlantic cold phases abstract during the last glacial period polar ice cores indicate climate asynchrony between the poles at the millennial time scale yet surface ocean circulation in large parts of the globe varied in tune with greenland temperature fluctuations suggesting that any anti phase behavior to a substantial degree must lie in the deeper global ocean circulation which is poorly understood outside the atlantic ocean here we present data from the north western indian ocean which indicate that the timing of maxima in northward extensions of glacial antarctic intermediate water gaaiw coincides with dramatically reduced thermohaline overturn in the north atlantic associated with the heinrich ice surge events he the repeated expansion of the gaaiw during hes recorded far north of the equator in the arabian sea suggests that southern hemisphere driven intermediate water mass variability forms an integral part of the inter hemisphere asynchronous climate change behavior at the millennial time scale

testing the 231pa 230th paleocirculation proxy a data versus 2d model comparison 1 variations of the atlantic meridional overturning circulation amoc are believed to have crucially influenced earth s climate due to its key role in the inter hemispheric redistribution of heat and carbon to assess its past strength the sedimentary 231pa 230th proxy has been developed and improved but also contested due to its sensitivity to other factors beyond ocean circulation in order to provide a better basis for the understanding of the atlantic 231pa 230th system and therefore to shed light on the controversy we compare new measurements of holocene sediments from the north brazilian margin to water column data and the output of a two dimensional scavenging circulation model based on modern circulation patterns and reversible scavenging parameters we show that sedimentary 231pa 230th data from one specific area of the atlantic are in very good agreement with model results suggesting that sedimentary 231pa 230th is predominantly driven by the amoc therefore 231pa 230th represents an appropriate method to reconstruct past amoc at least qualitatively along the western margin

accelerated drawdown of meridional overturning in the late glacial atlantic triggered by transient pre h event freshwater perturbation 1 abrupt decreases of the atlantic meridional overturning circulation moc during the late pleistocene have been directly linked to catastrophic discharges of glacimarine freshwater triggering disruption of northward marine heat transport and causing global climate changes here we provide measurements of excess sedimentary 231 pa 230 th from a high accumulation sediment drift deposit in the ne atlantic that record a sequence of sudden variations in the rate of moc associated deep ocean ventilation and surface ocean climatology the data series reveal a sequential decrease in the moc rate at 18 0 ka bp ago that coincides with only transient and localized freshwater inputs this change represents a substantial though not total cessation in moc that predates the major heinrich h1 meltwater event by at least 1 200 years these results highlight the potential of targeted freshwater perturbations in promoting substantial moc changes without a direct linking with catastrophic freshwater surges citation hall i r s b moran r zahn p c knutz c c shen and r l edwards 2006 accelerated drawdown of meridional overturning in the late glacial atlantic triggered by transient pre h event freshwater perturbation geophys res lett 33 l16616 doi 10 1029 2006gl026239

centennial to millennial scale ice ocean interactions in the subpolar northeast atlantic 18 41 kyr ago in order to monitor the evolution of the british irish ice sheet biis and its influence in surface ocean structure during marine isotopic stages mis 2 and 3 we have analyzed the sediments recovered in core md04 2829cq rosemary bank north rockall trough northeast atlantic dated between 41 and 18 ka b p ice rafted debris flux and composition 40ar 39ar ages of individual hornblende grains multispecies planktonic stable isotope records planktonic foraminifera assemblage data and faunal based sea surface temperatures ssts demonstrate a close interaction between biis dynamics and surface ocean structure and water properties in this region the core location lies beneath the north atlantic current nac and is ideal for monitoring the shifts in the position of its associated oceanic fronts as recorded by faunal changes these data reveal a succession of biis sourced iceberg calving events related to low sst usually synchronous with dramatic changes in the composition of the planktonic foraminifera assemblage and with variations in the stable isotope records of the taxa neogloboquadrina pachyderma sinistral coiling and globigerina bulloides the pacing of the calving events from typically dansgaard oeschger millennial timescales during late mis 3 to multicentennial cyclicity from 28 ka b p represents the build up of the biis and its growing instability toward heinrich event he 2 and the last glacial maximum our data confirm the strong coupling between biis instabilities and the temperature and salinity of surface waters in the adjacent northeast atlantic and demonstrate the biis s ability to modify the nac on its flow toward the nordic seas in contrast subsurface water masses were less affected except during the greenland stadials that contain hes when most intense water column reorganizations occurred simultaneously with the deposition of cream colored carbonate sourced from the laurentide ice sheet

south atlantic intermediate water advances into the north east atlantic with reduced atlantic meridional overturning circulation during the last glacial period the nd isotopic composition end of seawater and cold water coral cwc samples from the gulf of cadiz and the alboran sea at a depth of 280 827 m were investigated in order to constrain middepth water mass dynamics within the gulf of cadiz over the past 40 ka end of glacial and holocene cwc from the alboran sea and the northern gulf of cadiz reveals relatively constant values 8 6 to 9 0 and 9 5 to 10 4 respectively such values are similar to those of the surrounding present day middepth waters from the mediterranean outflow water mow end 9 4 and mediterranean sea water msw end 9 9 in contrast glacial end values for cwc collected at thermocline depth 550 827 m in the southern gulf of cadiz display a higher average value 8 9 0 4 compared to the present day value 11 7 0 3 this implies a higher relative contribution of water masses of mediterranean msw or south atlantic origin east antarctic intermediate water eaaiw our study has produced the first evidence of significant radiogenic end values 8 at 19 23 24 and 27 ka which are coeval with increasing iceberg discharges and a weakening of atlantic meridional overturning circulation amoc since mow end values remained stable during the last glacial period it is suggested that these radiogenic end values most likely reflect an enhanced northward propagation of glacial eaaiw into the eastern atlantic basin

thermohaline instability in the north atlantic during meltwater events stable isotope and ice rafted detritus records from core so75 26kl portuguese margin a benthic isotope record has been measured for core so75 26kl from the upper portuguese margin 1099 m water depth to monitor the response of thermohaline overturn in the north atlantic during heinrich events evaluating benthic 18o in ts diagrams in conjunction with equilibrium c fractionation implies that advection of mediterranean outflow water mow to the upper portuguese margin was significantly reduced during the last glacial 15 compared to 30 today the benthic isotope record along core so75 26kl therefore primarily monitors variability of glacial north atlantic conveyor circulation the 14c accelerator mass spectrometry ages of 13 54 07 and 20 46 12 ka for two ice rafted detritus ird layers in the upper core section and an interpolated age of 36 1 ka for a third ird layer deeper in the core are in the range of published 14c ages for heinrich events h1 h2 and h4 marked depletion of benthic 13c by 0 7 1 1 during the heinrich events suggests reduced thermohaline overturn in the north atlantic during these events close similarity between meltwater patterns inferred from planktonic 18o at site 609 and ventilation patterns inferred from benthic 13c in core so75 26kl implies coupling between thermohaline overturn and surface forcing as is also suggested by ocean circulation models benthic 13c starts to decrease 1 5 2 5 kyr before heinrich events hl and h4 fully increased values are reached 1 5 3 kyr after the events indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor s full strength well after the events benthic 13c changes in the course of the heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation a distinct feature of thermohaline instability in numerical models inferrred gradual spin up of thermohaline circulation after hl and h4 is in contrast to abrupt wanning in the north atlantic region that is indicated by sudden increases in greenland ice core 18o and in marine faunal records from the northern north atlantic from this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the greenland ice core record

common climatic signal from glaciers in the european alps over the last 50 years conventional glacier wide mass balances are commonly used to study the effect of climate forcing on glacier melt unfortunately the glacier wide mass balances are also influenced by the glacier s dynamic response investigations on the effects of climate forcing on glaciers can be largely improved by analyzing point mass balances using a statistical model we have found that 52 of the year to year deviations in the point mass balances of six glaciers distributed across the entire european alps can be attributed to a common variability point mass balance changes reveal remarkable regional consistencies reaching 80 for glaciers less than 10 km apart compared to the steady state conditions of the 1962 1982 period the surface mass balance changes are 0 85 m water equivalent w e a 1 for 1983 2002 and 1 63 m w e a 1 for 2003 2013 this indicates a clear and regionally consistent acceleration of mass loss over recent decades over the entire european alps

glacial interglacial circulation changes inferred from 231pa 230th sedimentary record in the north atlantic region 1 studies from the subtropical western and eastern atlantic ocean using the 231pa 230th ratio as a kinematic proxy for deep water circulation provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation moc over the last deglaciation in this study we present a compilation of existing and new sedimentary 231pa 230th records from north atlantic cores between 1710 and 4550 m water depth comparing sedimentary 231pa 230th from different depths provides new insights into the evolution of the geometry and rate of deep water formation in the north atlantic during the last 20 000 years the 231pa 230th ratio measured in upper holocene sediments indicates slow water renewal above 2500 m and rapid flushing below consistent with our understanding of modern circulation in contrast during the last glacial maximum lgm glacial north atlantic intermediate water gnaiw drove a rapid overturning circulation to a depth of at least 3000 m depth below 4000 m water renewal was much slower than today at the onset of heinrich event 1 transport by the overturning circulation declined at all depths gnaiw shoaled above 3000 m and significantly weakened but did not totally shut down during the bolling allerod ba that followed water renewal rates further decreased above 2000 m but increased below our results suggest for the first time that ocean circulation during that period was quite distinct from the modern circulation mode with a comparatively higher renewal rate above 3000 m and a lower renewal rate below in a pattern similar to the lgm but less accentuated moc during the younger dryas appears very similar to ba down to 2000 m and slightly slower below

surface melt induced acceleration of greenland ice sheet flow ice flow at a location in the equilibrium zone of the west central greenland ice sheet accelerates above the midwinter average rate during periods of summer melting the near coincidence of the ice acceleration with the duration of surface melting followed by deceleration after the melting ceases indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice bedrock interface interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting with larger increases accompanying higher amounts of summer melting the indicated coupling between surface melting and ice sheet flow provides a mechanism for rapid large scale dynamic responses of ice sheets to climate warming

a pb isotope tracer of ocean ice sheet interaction the record from the ne atlantic during the last glacial interglacial cycle abstract ice sheet ocean interactions are both a response to climate forcing and a source of climate feedback releasing freshwater to the surface ocean and influencing climate and atmospheric co2 through changes in ocean circulation documenting the outcomes of these interactions for recent glacial cycles is important given current and future scenarios of polar ice retreat however this is currently hampered by lack of accurate constraints on ice sheet development and demise marine sedimentary pb isotope records have potential to investigate these aspects of ice sheet feedbacks at high temporal resolution because of the sensitivity of the pb isotope composition to continental weathering intensity and solute flux here we present a pb isotope record sourced from the femn oxyhydroxide fraction in marine sediments from odp site 980 on feni drift 2168 mbsl rockall trough ne atlantic spanning the last 43 ka the location of site 980 at the northern edge of the british irish ice sheet biis makes it well placed to monitor changes in biis development as it responded to migration of the polar front during the last glacial interglacial cycle the data reveal millennial scale cyclicity in pb isotope composition reminiscent of dansgaard oeschger cycles from the start of the record until heinrich event 2 43 24 ka characterised by extreme shifts to radiogenic compositions i e variation in 206pb 204pb from 18 9 to 20 5 the period 24 17 5 ka is also characterised by exceptionally radiogenic and highly variable pb isotope compositions associated with the rapid and repeated expansion and collapse of the biis the presence of such radiogenic pb isotope compositions during periods of maximum ice sheet activity support interpretation of the subglacial environment as an active weathering environment contributing to biogeochemical cycles through the transport vectors of meltwater release and debris laden ice calving

u th and pa insights into sedimentological and paleoceanographic changes off hudson strait labrador sea during the last 37 ka with special attention to methodological issues 231 pa and 230 th excess calculation abstract a 9 m long sediment core spanning the last 37 ka has been raised from the lower labrador continental slope off the hudson strait shelf edge it has been analyzed for its u th and pa isotope contents along with current sedimentological parameters as a means to retrieve information about sedimentological changes in response to northeastern laurentide ice sheet lis margin instabilities the sequence yielded a high resolution record of subglacial detrital carbonate pulses from hudson strait assigned to heinrich events h2 and h1 whereas h0 was missing large variations in bulk sediment u and th contents as well as in 234 u 238 u activity ratio are observed throughout the sequence leading to large uncertainties when calculating excesses in 231 pa and 230 th 231 paxs and 230 thxs over their supported and in growth

phasing of deglacial warming and laurentide ice sheet meltwater in the gulf of mexico evidence is emerging that the tropical climate system played a major role in global climate change during the last deglaciation however existing studies show that deglacial warming was asyn chronous across the tropical band complicating the identification of causal mechanisms the orca basin in the northern gulf of mexico is ideally located to record subtropical atlantic sea surface temperature sst warming in relation to meltwater input from the laurentide ice sheet paired d 18 o and mg ca data on the planktonic foraminifer globigerinoides ruber from core en32 pc6 are used to separate deglacial changes in sst and d 18 o of sea water sst as calculated from mg ca data increased by 3 8c from ca 17 2 to 15 5 ka in association with heinrich event 1 and was not in phase with greenland air temperature subtracting tem perature effects from d 18 o values in g ruber reveals two excur sions representing laurentide meltwater input to the gulf of mex ico one of 1 5 from ca 16 1 to 15 6 ka and a second major spike of 2 5 from ca 15 2 to 13 0 ka that encompassed melt water pulse 1a and peaked ca 13 8 ka during the bolling allerod conversion to salinity through the use of a laurentide meltwater end member of 225 indicates that near surface salinity de creased by 2 4 during these spikes these results suggest that gulf of mexico sst warming preceded peak laurentide ice sheet decay and the bolling allerod interval by 2 k y and that heat was retained in the subtropical atlantic during heinrich event 1 consistent with modulation of deglacial climate by thermohaline circulation

high frequency climate fluctuations over the last deglaciation in the alboran sea western mediterranean evidence from calcareous plankton assemblages abstract a high resolution study with a centennial scale resolution has been performed on the calcareous plankton assemblage coccolithophores and planktonic foraminifera at ocean drilling program site 976 alboran sea western mediterranean focusing on the interval between 20 and 9 ka in order to reconstruct changes in surface and subsurface water dynamics and productivity the biotic surface water proxies integrate the extremely detailed multi decadal scale geochemical data set and the pollen record already available at the core thus providing a complete paleoenvironmental paleoceanographic reconstruction the results highlight the sensitivity of the calcareous plankton in recording stadial interstadial phases and higher frequency climatic events that produced changes in sea water features wind induced upwelling and river discharge during the last glacial period favored nutrient availability and moderate productivity during heinrich stadial 1 hs1 the arrival of cold and fresher waters from iceberg melting in the north atlantic significantly hampered productivity at its lowest values of the whole investigated interval calcareous plankton behavior supports the hypothesis that hs1 is a composite event marked by three phases hs1a characterized by the coldest sst polar atlantic water inflow into the basin and a first step toward a dry climate on the continent hs1b recording enhanced freshwater inflow and drought on land and hs1c indicating reduced polar water influx the bolling allerod ba interstadial sees the highest rate of productivity in the entire interval and accompanies the deposition of organic rich layer 1 during the ba coccolithophore assemblage also marks higher frequency changes in hydrographic conditions apparently in relation with greenland interstadial i gi i high variability the younger dryas stadial yd is characterized by refunding cold and dry conditions although not as severe as during hs1 and by enhanced detrital input in surface water during drier conditions on land the holocene onset records climate amelioration both in the marine and continental environment although coccolithophore productivity is not as high as during the ba the rapid shifts in climate sensitive taxa support the hypothesis that each of the climate phases subphases is characterized by a complex pattern forced by strong interaction between ice sheet instability meltwater discharge entering the gibraltar strait north atlantic westerlies and moisture availability clearly impacting over surface water temperature productivity and land masses

multidecadal ocean variability and nw european ice sheet surges during the last deglaciation a multiproxy paleoceanographic record from the atlantic margin off the british isles reveals in unprecedented detail discharges of icebergs and meltwater in response to sea surface temperature increases across the last deglaciation we observe the earliest signal of deglaciation as a moderate elevation of sea surface temperatures that commenced with a weakly developed thermocline and the presence of highly ventilated intermediate waters in the rockall trough this warming pulse triggered a series of multidecadal ice rafted debris peaks that culminated with a major meltwater discharge at 17 500 years before present related to ice sheet disintegration across the nw european region the impact of meltwater caused a progressive reduction in deep water ventilation and a sea surface cooling phase that preceded the collapse of the laurentide ice sheet during heinrich event 1 by 500 1000 years a similar sequence of rapid ocean ice sheet interaction across the european continental margin is identified during the bolling allerod to younger dryas transition the strategic location of our sediment core suggests a sensitive and rapid response of ice sheets in nw europe to transient increases in thermohaline heat transport

rapid climate transitions in a coupled ocean atmosphere model recent geochemical data1 2 have challenged the view that rapid climate fluctuations in the north atlantic at the end of the last glacial were caused by the thermohaline circulation of the ocean being switched on or off instead these data suggest that the circulation pattern must have switched between a warm deep mode and a cold shallow mode probably associated with different sites of deep convection3 here i present simulations with a three dimensional ocean model coupled to an idealized atmosphere which show this kind of transition the mechanism for the transition is a rearrangement of convection in the north atlantic triggered by a brief freshwater pulse this results in a drop in sea surface temperature in the north atlantic by up to 5 c within less than 10 years the rate of north atlantic deep water nadw forma tion is the same in the cold climate as in the warm climate but nadw sinks to intermediate depths only while antarctic bottom water pushes north to fill the entire abyssal atlantic

suborbital intermediate water variability inferred from paired benthic foraminiferal cd ca and δ13c in the tropical west atlantic and linking with north atlantic climates abstract paired benthic cd ca and 13c records have been generated along core m35003 in the western tropical atlantic decreased glacial water column dissolved cadmium cdw and increased benthic 13c indicate enhanced ventilation with nutrient deplete intermediate waters in line with similar inferences from other north atlantic mid depth records an abrupt early deglacial 13c collapse that is associated with a marked positive cdw anomaly indicates a transient collapse of mid depth ventilation from north atlantic sources conceivably in conjunction with the h1 meltwater anomaly the cdw record displays fine scale fluctuations that mimic the greenland dansgaard oeschger d o cycles and show decreased cdw during stadials this pattern is opposite to cdw variations in a deep water record from bermuda rise that display increased cdw concentrations during stadials the divergent pattern between mid depth and deep water cdw records indicates millennial scale switches between deep and shallow convection in the glacial north atlantic at the pace of the d o climatic cycles several high amplitude anomalies occur in the cdw record that reach levels similar to those observed today in the north pacific while a substantial nutrient increase in the mid depth north atlantic cannot be ruled out during these events changes of pore water chemistry and cd p fractionation during biological uptake offer alternative scenarios to explain the peak cdw maxima

surface water conditions and calcium carbonate preservation in the fram strait during marine isotope stage 2 28 8 15 4 kyr 1 we present a high resolution record of calcium carbonate preservation alongside the distribution pattern of planktic foraminifera from the fram strait the record covers the marine isotope stage mis 2 28 8 to 15 4 kyr including the last glacial maximum lgm and the early deglaciation in multidecadal temporal resolution the investigation is based on the distribution patterns of planktic foraminifera stable isotopes mean shell weight of neogloboquadrina pachyderma the degree of fragmentation of planktic shells caco3 content and geochemical and sedimentological data the dissolution proxies indicate long lasting periods of markedly reduced preservation of calcium carbonate at 28 8 27 2 24 1 23 2 and 16 6 15 4 kyr in addition to a number of short lasting periods 300 year during the lgm these periods are accompanied by a simultaneous and significant reduction in specimens of subpolar planktic foraminifera indicating a southward expansion of arctic surface water masses prolonged periods with high abundances of subpolar foraminiferal species and good preservation of calcium carbonate are attributed to increased influence of atlantic water masses which appears to persist throughout most of the mis 2 comparison to holocene mis 1 shell weight records shows that the preservation during mis 2 was overall better even during the events of dissolution this was probably a reflection of the low concentration of atmospheric co2 at the time

did north atlantic overturning halt 17 000 years ago models indicate that a complete shutdown of deep and intermediate water production is a possible consequence of extreme climate conditions in the northern north atlantic and the high ratio of 231pa to 230th on bermuda rise is evidence that this might have happened 17 ka during heinrich event 1 h1 however new radiocarbon data from bivalves that lived at 4 6 km on the bermuda rise during h1 lead to a different conclusion the bivalve data do indeed indicate ventilation of the deep western north atlantic was suppressed during h1 but not as much as it was during the last glacial maximum we propose that high diatom flux to the bermuda rise during h1 is at least in part responsible for increased 231pa 230th at that time although we cannot say for sure why opal production was so high in a gyre center location at that time increased leakage of silica rich waters from the southern ocean to the north atlantic is one possibility

neogloboquadrina pachyderma sinistral coiling as paleoceanographic tracers in polar oceans evidence from northeast water polynya plankton tows sediment traps and surface sediments the abundance and chemistry of the planktonic foraminifera neogloboquadrina pachyderma sinistral coiling have long been used as tools for monitoring polar surface ocean changes and for correlating these changes to atmospheric and thermohaline circulation fluctuations however due to its remote habitat very little is known about how modern n pachyderma s respond to changing environmental conditions in the polar seas modern samples of n pachyderma s from the northeast water polynya provide a means for studying how environmental conditions affect the vertical distribution and chemistry of this species highest abundances of n pachyderma s were associated with the chlorophyll maximum in the surface 20 80 m where they are exploiting their primary food source evidence suggests that the addition of a calcite crust modifies the calcite tests of some n pachyderma s between 50 and 200 m increasing shell density and modifying shell chemistry the shell mass of encrusted forms is 3 4 times greater than the nonencrusted forms between 50 and 200 m the oxygen isotope composition of n pachyderma s shells increase by 1 5 in response to local water column gradients the 13c values of n pachyderma s are basically invariant with depth in this region are consistently 1 0 depleted in comparison with the 13c for equilibrium calcite and remain basically constant during the shell thickening process mass balance calculations suggest that encrustation occurs at all depths but abundance counts suggest that the process occurs mostly at the depth of the main pycnocline sediment fluxes of n pachyderma s occur during a 2 week bloom event and decrease to almost zero below complete ice cover the decoupling of the processes controlling abundances and shell chemistry explain the discrepancies between transfer function and isotopically derived paleotemperature estimates of surface conditions in some oceanic settings the ability of 18o to record surface ocean conditions will depend on vertical water column gradients as evidenced by the differences in core top calibrations between the north and south atlantic

evidence from the high latitude north atlantic for variations in antarctic intermediate water flow during the last deglaciation we present evidence that the characteristic chemical signature based on coupled benthic foraminiferal cd ca and d13c of antarctic intermediate waters aaiw penetrated throughout the intermediate depths of the atlantic basin to the high latitude north atlantic during the abrupt cooling events of the last deglaciation heinrich 1 and the younger dryas aaiw may play the dynamic counterpart to the bipolar seesaw when near freezing salty bottom waters from the antarctic aabw sluggishly ventilate the deep ocean our data reinforce the concept that interglacial circulation is stabilized by salinity feedbacks between salty northern sourced deep waters nadw and fresh southern sourced waters aabw and aaiw further the glacial ocean may be susceptible to the more finely balanced relative densities of nadw and aaiw due to either freshwater input or a reversal of the salinity gradient such that the ocean is poised for nadw collapse via a negative salinity feedback the unstable climate of the glacial period and its termination may arise from the closer competition for ubiquity at intermediate depths between northern and southern sourced intermediate waters

controls on seawater 231pa 230th and 232th concentrations along the flow paths of deep waters in the southwest atlantic measurements of dissolved th 230 pa 231 and th 232 were made for twelve full depth profiles along a southwest atlantic section during geotraces cruise ga02s sampling captures all the main atlantic deep water masses along their meridional flow paths and allows insight into the control on th and pa in a setting where waters are flowing in opposing directions with direct relevance to understanding the use of pa 231 th 230 as an ocean circulation proxy water column th 230 increases linearly with depth in line with expected reversible scavenging models pa 231 increases from the surface to similar to 1200 1500 m but is invariant or decreases with greater depth deviating from the behavior expected for reversible scavenging dissolved pa 231 th 230 ratios display a mid water column maximum at similar to 1000 2000 m which is broadly coincident with upper circumpolar deep water below 2000 m nuclide distributions and ratios exhibit no dependence on water mass nor any indication of progressive change within a water mass challenging the use of pa 231 th 230 as a past circulation tracer in the south atlantic calculation of horizontal transport of th 230 and pa 231 by ocean circulation indicates a net southward export out of the atlantic of 19 of the pa 231 and 3 of the th 230 produced in that ocean this removal is all from the north atlantic while in the south atlantic removal to sediment equals production simple one dimensional modeling can simulate th 230 profiles but not the mid water column maximum observed in pa 231 profiles suggesting an additional source of pa 231 perhaps lateral transport from the margin or removal at depth due to bottom scavenging near seafloor minima in concentrations indicates bottom scavenging of th 230 and 231 pa which is enhanced in the presence of nepheloid layers particularly for 231pa this additional scavenging fractionates th 230 and pa 231 and in the presence of nepheloid layers may lead to an increase in sedimentary pa 231 th 230 ratios th 232 concentrations were paired with th 230 derived residence times in the upper 250 m of the water column to test the application of th as a tracer of dust deposition maxima in th 232 indicate high dust input from the african and possibly south american continents

freshwater distribution in the arctic ocean simulation with a high resolution model and model data comparison 1 a high resolution numerical simulation of the arctic ocean is analyzed in order to study the fate of river runoff and freshwater fluxes in the arctic ocean the model is driven by realistic winds and thermodynamic forcing from the european centre for medium range weather forecasting ecmwf reanalysis data set dye tracers have been added to visualize the pathways followed by low salinity water from the major arctic rivers and bering strait inflow the model is spun up using repeated forcing with the 1979 annual cycle for 20 years then the 1979 through 1998 atmospheric forcing is applied under the influence of the 1979 through early 1980s winds a large plume of river runoff exits the broad eurasian shelf seas along the lomonosov ridge starting in about 1985 the locus of shelf to basin transport shifts eastward to the alpha mendeleyev ridge complex this shift in the model output is related to changes in the sea surface height ssh fields which we attribute primarily to shifts in surface wind stresses model resolution runoff inputs and relaxation terms in the lena river delta region are analyzed in detail to expose issues with model performance at boundaries with freshwater inflow suggestions are made for improving future simulations of river runoff in basin scale models

the origin of heinrich layers evidence from h2 for european precursor events recent well dated isotopic sr nd fingerprinting of heinrich layer ice rafted detritus ird on the european margin indicates supply from european ice sheets as precursors to laurentide ice sheet lis supply f e grousset et al geology 28 2000 123 126 h snoeckx et al mar geol 158 1999 197 208 these precursorevents lead lis input by up to 1 5 ka f e grousset et al geology 28 2000 123 126 and have been interpreted to indicate lis collapse during heinrichevents stimulated by events originating on the european side of the atlantic f e grousset et al geology 28 2000 123 126 such phasing of ird supply from different sources within heinrich layers therefore has implications for the origin and mechanics of heinrichevents we present evidence here that the ird comprising heinrich layer 2 h2 20 21 14c ka bp on the european continental margin contains detrital campanian upper chalk deriving from bedrock sources eroded on the celtic shelf by the british ice sheet bis in addition to lithic material sourced from the lis high resolution radiocarbon chronology indicates chalk grain deposition as discrete pulses both before and coincident with supply of lis sourced detritus the specificity of the chalk fingerprint to the bis enables a 700 1000 yr lag between the bis and lis events to be identified this phasing indicates a more rapid response of the outlet lobes draining the smaller bis than those draining the lis and implicates external climatic forcing of heinrichevents it is unlikely that this precursorevent represents ird event 18 the recently identified 1 2 ka ird cycle event which immediately precedes h2 because the lag between precursor and main event is here less than 1 5 ka and because such pervasive periodicity is not apparent in european continental margin ird records the later synchroneity between the bis and lis input in h2 identifies glacio eustatic sea level rise associated with lis discharges as a possible feedback mechanism causing destabilisation of ice streams elsewhere during heinrichevents

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