response to comment on mixed layer deepening during heinrich events a multi planktonic foraminiferal δ18o approach hillaire marcel and de vernal question our hypothesis of the homogenization of the upper water masses of the north atlantic during heinrich events 0 1 and 4 here we respond to their specific points and synthesize data from the southern labrador sea cores the basic tenet of hillaire marcel and de vernal s arguments

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

millennial scale changes in north atlantic circulation since the last glaciation ocean circulation is closely linked to climate change on glacial interglacial and shorter timescales extensive reorganizations in the circulation of deep and intermediate depth waters in the atlantic ocean have been hypothesized for both the last glaciation1 2 3 4 5 6 and the subsequent younger dryas cold interval3 6 7 8 9 10 but there has been little palaeoceanographic study of the subtropical gyres11 12 13 these gyres are the dominant oceanic features of wind driven circulation and as such they reflect changes in climate and are a significant control on nutrient cycling and possibly atmospheric co2 concentrations here we present cd ca ratios in the shells of benthic foraminifera from the bahama banks that confirm previous suggestions11 12 that nutrient concentrations in the north atlantic subtropical gyre were much lower during the last glacial maximum than they are today up to 50 lower according to our data these contrasting nutrient burdens imply much shorter residence times for waters within the thermocline of the last glacial maximum below the glacial thermocline nutrient concentrations were reduced owing to the presence of glacial north atlantic intermediate water a high resolution cd ca record from an intermediate depth indicates decreased nutrient concentrations during the younger dryas interval as well mirroring opposite changes at a nearby deep site3 9 together these observations suggest that the formation of deep and intermediate waters north atlantic deep water and glacial north atlantic intermediate water respectively wax and wane alternately on both orbital and millennial timescales

the u series toolbox for paleoceanography the geochemistry of marine sediments is a major source of information about the past environment of the many measurements that provide such information those of the u series nuclides are unusual in that they inform us about the rate and timescales of processes oceanic processes such as sedimentation productivity and circulation typically occur on timescales too short to be assessed using parent daughter isotope systems such as rb sr or sm nd so the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides principally 14c or the u series nuclides this makes the u series nuclides powerful allies in the quest to understand the past ocean climate system and has led to their widespread application over the last decade as in other applications of the u series those in paleoceanography rely on fractionation of the nuclides away from secular equilibrium in the oceanic setting this fractionation is generally due to differences in the solubility of the various nuclides the behavior of the u series nuclides in the ocean environment was widely researched in the middle decades of the twentieth century this work established knowledge of the concentrations of the nuclides in the various compartments of the ocean system and of their fluxes between these compartments such understanding was comprehensively summarized in the ivanovich and harmon u series volume 1992 particularly by cochran 1992 understanding of the behavior of the u series nuclides has not advanced very dramatically in the decade since that summary but a major theme of research has been the use of this geochemical understanding to develop u series tools to assess the past environment table 1 view this table table 1 summary of seawater data for u series nuclides with paleoceanographic applications s soluble i insoluble full descriptions of the paleoceanographic uses and references are provided in the text further details of the half

thorium 230 profiling in deep sea sediments high resolution records of flux and dissolution of carbonate in the equatorial atlantic during the last 24 000 years variations in carbonate flux and dissolution which occurred in the equatorial atlantic during the last 24 000 years have been estimated by a new approach that allows the point by point determination of paleofluxes to the seafloor an unprecedented time resolution can thus be obtained which allows sequencing of the relatively rapid events occurring during deglaciation the method is based on observations that the flux of unsupported 230th into deep sea sediments is nearly independent of the total mass flux and is close to the production rate thus excess 230th activity in sediments can be used as a reference against which fluxes of other sedimentary components can be estimated the study was conducted at two sites ceara rise western equatorial atlantic and sierra leone rise eastern equatorial atlantic in cores raised from three different depths at each site from measurements of 230th and caco3 changes in carbonate flux with time and depth were obtained a rapid increase in carbonate production starting at the onset of deglaciation was found in both areas this event may have important implications for the postglacial increase in atmospheric co2 by increasing the global carbonate carbon to organic carbon rain ratio and decreasing the alkalinity of surface waters and possibly the north atlantic deep water increased carbonate dissolution occurred in the two regions during deglaciation followed by a minimum during mid holocene and renewed intensification of dissolution in late holocene during the last 16 000 years carbonate dissolution was consistently more pronounced in the western than in the eastern basin reflecting the influence of antarctic bottom water in the west this trend was reversed during stage 2 possibly due to the accumulation of metabolic co2 below the level of the romanche fracture zone in the eastern basin

boundary scavenging at the east atlantic margin does not negate use of 231pa 230th to trace atlantic overturning abstract the 231 pa 230 th method is a promising tool to reconstruct ocean circulation over the past glacial interglacial cycle however marine particle flux may constrain the applicability of this ratio as a direct quantitative proxy for the strength of atlantic meridional overturning circulation amoc by influencing the fractionation between the in situ produced 231 pa and 230 th in ocean water here we present 231 pa 230 th down core profiles from high particle flux areas off namibia and senegal covering the past 35 ka the 231 pa 230 th profiles at these sites show very different responses to temporal variations of particle fluxes and to changes in water masses our results show that sedimentary 231 pa 230 th in the eastern atlantic margin is linked to particle flux but controlled primarily by the mode of the amoc our data suggest that during the past 30 ka the high productivity eastern margin was not capable of importing and storing significant amounts of 231 pa from the open ocean consequently the applicability of the 231 pa 230 th proxy to reconstruct past ocean circulation is not hampered by this potentially additional 231 pa sink

evolution of the deep atlantic water masses since the last glacial maximum based on a transient run of ncar ccsm3 during the last deglaciation from approximately 21 to 11 thousand years ago the high latitudes of the atlantic ocean underwent major changes besides the continuous warming the polar and subpolar ocean surface received a large amount of meltwater from the retracting ice sheets these changes in temperature and salinity affected deep waters such as the antarctic bottom water aabw and the north atlantic deep water nadw which are formed in the southern ocean and in the northern north atlantic respectively in this study we present the evolution of the physical properties and distribution of the aabw and the nadw since the last glacial maximum using the results of a transient simulation with ncar ccsm3 in this particular model scenario with a schematic freshwater forcing we find that modern nadw with its characteristic salinity maximum at depth was absent in the beginning of the deglaciation while its intermediate version glacial north atlantic intermediate water gnaiw was being formed gnaiw was a cold and relatively fresh water mass that dominated intermediate depths between 60 and 20 n at this time most of the deep and abyssal atlantic basin was dominated by aabw within the onset of the bolling allerod period at nearly 15 thousand years ago ka gnaiw expanded southwards when the simulated meridional overturning circulation overshoots the transition between gnaiw and nadw ocurred after that when aabw was fresh enough to allow nadw to sink deeper in the water column when the nadw appears 11 ka aabw retracts and is constrained to lie near the bottom

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

a geochemical criterion for the recognition of heinrich events and estimation of their depositional fluxes by the 230thexcess 0 profiling method abstract a core from the armorican seamount 46 23 8 n 12 32 8 w 3849 m in the deep ne atlantic southwest of the british isles exhibits unusual mg al ratios in the sediments deposited during the last glacial period high mg al ratio values are restricted within layers of 14 cm thick and mark the presence of ice rafted detrital material deposited by heinrich events h1 h2 and h4 these peaks in the ratio are caused by local increases in the dolomite quartz and feldspar content and concomitant dilutions in the phyllosilicate content which overall increase mg and decrease al the known times of the heinrich events provide a chronology for the core and indicate a considerably slower mean sediment accumulation rate during and after the glacial interglacial transition 2 2 cm ky 1 than in earlier glacial times mean 5 6 cm ky 1 within the glacial however 230 th excess 0 profiling demonstrates that accumulation fluxes were at least 7 higher than normal during the deposition of heinrich events up to 25 g cm 2 ky 1 versus a background glacial level of 3 7 g cm 2 ky 1 because of post depositional bioturbation peak fluxes are likely to have been even higher the 230 th excess 0 method also reveals the presence of event h3 which bears minor dolomite and is the least well developed of the last four heinrich events despite the marked compositional and flux variations the total mass deposited at this site is estimated at only 2 4 4 9 and 7 9 g cm 2 for events h1 h2 and h4 respectively these inventories were deposited over maximum periods of 2 0 1 1 and 0 7 ky respectively but delivery could have been much more rapid of the order of a few hundred years

evidence for changes in the north atlantic deep water linked to meltwater surges during the heinrich events abstract deep sea sediment records from north atlantic cores 40 n 55 n provide evidence of several massive iceberg discharges known as heinrich events during the last glacial period high resolution benthic 18 o and 13 c records from north atlantic sediment cores were used to monitor the impact of heinrich events on thermohaline circulation and to estimate the sensitivity of deep oceanic circulation to changes in freshwater input to the north atlantic surface waters our data indicate that major rearrangements of deep water masses were directly associated with these massive iceberg discharges to trace in detail the deep water conditions in the north atlantic benthic 13 c values in several cores were used to generate time slices before during and after heinrich event 4 dated at 35 ka bp although north atlantic deep water continued to form during the oxygen isotope stage 3 at 37 ka bp deep circulation was characterized by an increased incursion of deep waters of southern origin which reduced the 13 c composition of north atlantic deep waters particularly in the eastern atlantic basin north atlantic deep water production was reduced during heinrich layer 4 hl4 at 35 ka bp synchronously with the changes in the surface water hydrology deep convection processes may have occurred in areas not affected by the salinity decrease soon after hl4 33 ka bp the 13 c distribution was similar to that before the event similarly a rapid return to initial 13 c values was observed at the end of the most clearly defined heinrich events hl5 hl4 and hl1 comparison between deep circulation patterns corresponding to hl4 the last glacial maximum and heinrich event 1 indicates that each of these periods was characterized by a different circulation state associated with changes in convection sites

size sorting in marine muds processes pitfalls and prospects for paleoflow speed proxies the basis for and use of fine grain size parameters for inference of paleoflow speeds is reviewed here the basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field experimental data on suspended sediment transport and deposition and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed in the deep sea sorting events occur under resuspension deposition events in benthic storms at flow speeds below 10 15 cm s 1 size in the noncohesive sortable silt 10 63 m range is controlled by selective deposition whereas above that range removal of finer material by winnowing also plays a role the best particle size instruments to measure a flow speed related grain size employ the settling velocity method while laser diffraction sizers can yield misleading results because of particle shape effects potential problems including source effects downslope supply on continental margins spatial variability of flow over bedforms and influence of ice rafted detritus are examined a number of studies using the sortable silt flow speed proxy are reviewed and inverse modeling of grain size distributions is examined outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed

changes in north atlantic deep water formation associated with the dansgaard oeschger temperature oscillations 60 10 ka we closely comparedhigh resolution d 13 c records of benthic foraminifera cibicides wuellerstorfi a proxy for deep water ventilation with the dansgaard oeschger temperature oscillations our results reveal different perturbations of deep water formation in the north atlantic ocean associated with the millennial scale climate oscillations during the last glacial period the cooling episodes associated with the drastic heinrich events are related to large reductions of deep water formation and a northward migration of 13 c depleted southern source deep waters to 621n in the north atlantic ocean the inter heinrich events which are correlatedto the other coldstad ials are markedby significant changes of sea surface temperature aroundthe rockall plateau variations in the flux of icebergs to the north atlantic ocean but are not associated with such important reduction of deep water formation if changes in the thermohaline circulation thc are associatedwith these millennial scale climatic oscillation they affect only the deeper water masses below 2000 m of the north atlantic ocean we thus show that equivalent degree of cooling over greenland is obtained with different perturbations of deep water formation our results either question the role of the thc as the unique explanation for these millennial scale climate oscillations or call upon an amplifying mechanism not yet taken into account r 2002 elsevier science ltd all rights reserved

deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation the degree of similarity of the 13c records of the planktonic foraminiferal species n pachyderma and of the benthic foraminiferal genus cibicides in the high latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle whereas continuous formation of deep water is recognized in the southern ocean the norwegian sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation however deep water formed in the north atlantic south of the norwegian sea during the last climatic cycle as early as isotope substage 5d and this area was also the only active northern source during stages 4 2 a detailed reconstruction of the geographic distribution of 13c in benthic foraminifera in the atlantic ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean whereas the glacial north atlantic deep water cannot be traced south of 40 n at shallower depth an oxygenated 13c rich intermediate water mass extended from 45 n to 15 s in the pacific ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700 2600 m and is best explained by stronger formation of intermediate water in high northern latitudes

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

fluxes of 230th and 231pa to the deep sea implications for the interpretation of excess 230th and 231pa 230th profiles in sediments abstract analysis of samples obtained with deep sea moored sediment traps deployed at 15 sites representing a wide range of oceanic conditions confirms that the flux of 230 th scavenged to the seafloor remains close to its production rate from the decay of 234 u in the overlying water column and generally validates the use of 230 th as a normalizing tool for paleoflux reconstruction after correction for trapping efficiency the flux of 230 th measured in the low flux regions amounts to 90 6 of the production rate with the notable exceptions of one site near the arabian sea upwelling and one site in the weddell sea a 230 th flux equivalent to 120 of the production rate was found in panama basin similar or more extensive scavenging of 230 th may be occurring at a pacific margin site off california and south of the polar front but these estimates are obscured by large errors on our trapping efficiency estimates in contrast the flux of 231 pa and the 231 pa 230 th ratio can vary strongly with particle flux following distinct trends in different oceanic basins in the atlantic ocean 231 pa fluxes and 231 pa 230 th are low and not sensitive to particle flux this is because of the short residence time of deep water in this basin resulting from thermohaline circulation which prevents the full development of lateral concentration gradients and full expression of boundary scavenging in the pacific ocean the sensitivity of 231 pa 230 th to particle flux is highest reflecting the longer residence time of deep water in the southern ocean 231 pa 230 th ratios are invariably high even when particle fluxes are low reflecting the predominance of opal which fractionates minimally between the two radionuclides interpretation of 231 pa 230 th recorded in sediments is thus complex in the atlantic this ratio is better suited for recording past changes in the strength of thermohaline circulation in the pacific it has the best potential for providing synoptic maps of past changes in particle flux in the southern ocean retrieval of information from 231 pa 230 th is more difficult and requires a more quantitative understanding of the influence of particle composition

glacial to interglacial changes in carbonate and clay sedimentation in the atlantic ocean estimated from 230th measurements abstract the cause of the climatically controlled fluctuations in the carbonate content of deep sea sediments remains the subject of uncertainty and debate three variables are involved supply of biogenic carbonate loss by dissolution and dilution by non carbonate phases it is suggested that 230 th which is produced in the ocean at a constant rate provides a reliable reference for measuring variations in rate of sedimentation on a regional scale results of a preliminary analysis based on published data indicate that for depths at and above the lysocline the carbonate fluctuations observed in cores from the north atlantic ocean are due primarily to variations in the terrigenous clay input which was 2 5 times higher during glacials than during interglacials carbonate deposition appears to have been somewhat reduced during glacials but probably not by more than a factor of 2 from published 230 th 232 th profiles it appears that the south atlantic ocean also received increased inputs of terrigenous clay during glacial periods

stress gradient coupling in glacier flow i longitudinal averaging of the influence of ice thickness and surface slope for a glacier flowing over a bed of longitudinally varying slope the influence of longitudinal stress gradients on the flow is analyzed by means of a longitudinal flow coupling equation derived from the vertically cross sectionally integrated longitudinal stress equilibrium equation by an extension of an approach originally developed by budd 1968 linearization of the flow coupling equation by treating the flow velocity u vertically averaged ice thickness h and surface slope in terms of small deviations u h and a from overall average datum values u 0 h 0 and 0 results in a differential equation that can be solved by green s function methods giving u x as a function of h x and x x being the longitudinal coordinate the result has the form of a longitudinal averaging integral of the influence of local h x and x on the flow u x u x u0 2l l 0 ln n nh n l exp x x l dx where the integration is over the length l of the glacier the operator specified deviations from the datum state and the term on which it operates which is a function of the integration variable x represents the influence of local h x x and channel shape factor x at longitudinal coordinate x on the flow u at coordinate x the influence being weighted by the influence transfer function exp x x l in the integral the quantity l that appears as the scale length in the exponential weighting function is called the longitudinal coupling length it is determined by rheological parameters via the relationship l 2h n n 3n where n is the flowlaw exponent n the effective longitudinal viscosity and n the effective shear viscosity of the ice profile n is an average of the local effective viscosity n over the ice cross section and n 1 is an average of n 1 that gives strongly increased weight to values near the base theoretically the coupling length l is generally in the range one to three times the ice thickness for valley glaciers and four to ten times for ice sheets for a glacier in surge it is even longer l 12h it is distinctly longer for non linear n 3 than for linear rheology so that the flow coupling effects of longitudinal stress gradients are markedly greater for non linear flow the averaging integral indicates that the longitudinal variations in flow that occur under the influence of sinusoidal longitudinal variations in h or with wavelength are attenuated by the factor 1 1 2 l 2 relative to what they would be without longitudinal coupling the short intermediate and long scales of glacier motion raymond 1980 over which the longitudinal flow variations are strongly partially and little attenuated are for 2l 2l 20l and 20l for practical glacier flow calculations the exponential weighting function can be approximated by a symmetrical triangular averaging window of length 4l called the longitudinal averaging length the traditional rectangular window is a poor approximation because of the exponential weighting the local surface slope has an appreciable though muted effect on the local flow which is clearly seen in field examples contrary to what would result from a rectangular averaging window tested with field data for variegated glacier alaska and blue glacier washington the longitudinal averaging theory is able to account semi quantitatively for the observed longitudinal variations in flow of these glaciers and for the representation of flow in terms of effective surface slope values exceptions occur where the flow is augmented by large contributions from basal sliding in the ice fall and terminal zone of blue glacier and in the reach of surge initiation in variegated glacier the averaging length 4l that gives the best agreement between calculated and observed flow pattern is 2 5 km for variegated glacier and 1 8 km for blue glacier corresponding to l h 2 in both cases if l varies with x but not too rapidly the exponential weighting function remains a fairly good approximation to the exact green s function of the differential equation for longitudinal flow coupling in this approximation l in the averaging integral is l x but is not a function of x effects of longitudinal variation of l are probably important near the glacier terminus and head and near ice falls the longitudinal averaging formulation can also be used to express the local basal shear stress in terms of longitudinal variations in the local stope stress with the mediation of longitudinal stress gradients

millennial scale variations in hydrography and biogeochemistry in the eastern equatorial pacific over the last 100 kyr abstract in this study we use records of nitrogen isotope ratios 15n uk 37 temperature estimates organic carbon and opal percentages from high resolution sediment cores located in the eastern equatorial pacific eep to explore the mechanisms linking millennial scale changes in low latitude sea surface temperature water column denitrification and surface productivity to the timing of northern or southern polar climate during the last 100 000 yr our results support a hypothesis that the southern hemisphere and its connection to the low latitudes via shallow subsurface ocean circulation has a primary influence on the biogeochemistry of the eep in addition our results suggest that during the last glacial stage denitrification rates fluctuated on millennial timescales in response to water column ventilation rather than upstream oxidant demand in intermediate depth waters however due to the poor age constraints available for marine isotopic stage mis 3 the eep sedimentary data presented here could support two conflicting mechanisms one driven by enhanced intermediate overturning circulation in the southern ocean during heinrich events antarctic warm events implying that subsurface flow rates control thermocline ventilation and a second one consistent with more sluggish intermediate circulation during antarctic warm events and giving a central role to the temperature control on oxygen solubility in southern ocean surface waters

reconstruction of the last deglaciation deconvolved records of δ18o profiles micropaleontological variations and accelerator mass spectrometric14c dating bioturbation acts as a low pass filter in displacing and reducing the amplitudes of stratigraphic signals this often leads to a loss of high frequency events in the stratigraphic record in addition when considering an isotopic signal 18o 14c measured in stratigraphic carriers such as foraminifera bioturbation and carrier abundance changes can create artifacts which may be falsely interpreted as leads or lags in the paleoclimatic record

short term flow dynamics of a retreating tidewater glacier leconte glacier alaska u s a an analysis of motion in the terminus region of leconte glacier alaska u s a delineates mechanisms that are important to tidewater glacier stability this glacier is undergoing rapid retreat since 1995 it has retreated 2 km and thinned 125 m at the present location of the terminus ice velocities in this region are quite high at the terminus they exceed 27 m d 1 our analysis reveals that fluctuations in speed and surface elevation are forced by ocean tides surface melt and precipitation the near terminus ice speed is 180 out of phase with the tide such that high tide corresponds to low speed smaller melt forced diurnal variations in speed are present throughout the lowermost 7 km speed ups in excess of 10 of the mean often occur after rainstorms as a result of increased basal water pressure and storage but the relation is not simple the velocity does not vary over the spring and summer seasons

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

dissolved and particulate 231 pa and 230 th in the atlantic ocean constraints on intermediate deep water age boundary scavenging and 231 pa 230 th fractionation 231pa and 230th concentrations were determined in filtered seawater and suspended particulate matter collected from the labrador sea and the equatorial and south atlantic to constrain their application as tracers of intermediate deep water age and atlantic thermohaline circulation distributions of total 231pa and 230th indicate the influence of recently formed north atlantic deep water as evidenced by nearly invariant concentrations below 1000 m in the labrador sea and increasing 231pa and 230th concentrations as deep waters progress southward from northern source regions application of a scavenging mixing model to both tracer distributions indicates an intermediate deep water age of 12 yr in the labrador sea and a 30 140 yr transit time to the low latitude stations we attribute a striking increase in total 230th in the labrador sea from 1993 to 1999 to aging of intermediate waters as a consequence of the cessation of deep convection in the labrador sea since 1993 the temporal change in the 230th age of these waters is consistent with the 6 yr time interval between the observations the average particulate 231pa 230th activity ratio in the labrador sea and low latitude deep waters is 0 057 0 003 significantly below the 231pa 230th production ratio 0 093 and in agreement with excess 231pa 230th ratios in holocene sediments 0 060 0 004 and sediment trap material 0 034 0 012 from the atlantic and model simulations this observation is consistent with the southward transport of deep water strongly attenuating boundary scavenging in the atlantic a latitudinal dependence in particle fractionation of these tracers is also evident with elevated fractionation factors fth pa observed near the equator and south atlantic gyre 11 compared to low values in the labrador sea 3 and southern ocean 2 there also exists a depth dependence in fth pa characterized by low values in surface waters a broad mid depth maximum and decreasing values towards the sea floor the latitudinal and depth variations in fth pa are suggested to reflect differences in the chemical composition of marine particles

how well would modern day oceanic property distributions be known with paleoceanographic like observations author s gebbie g streletz gj spero hj abstract 2016 american geophysical union all rights reserved compilations of paleoceanographic observations for the deep sea now contain a few hundred points along the oceanic margins mid ocean ridges and bathymetric highs where seawater conditions are indirectly recorded in the chemistry of buried benthic foraminiferal shells here we design an idealized experiment to test our predictive ability to reconstruct modern day seawater properties by considering paleoceanographic like data we attempt to reconstruct the known modern day global distributions by using a state estimation method that combines a kinematic tracer transport model with observations that have paleoceanographic characteristics when a modern like suite of observations practical salinity seawater 18o 13cdic po4 no3 and o2 is used from the sparse paleolocations the state estimate is consistent with the withheld data at all depths below 1500 m suggesting that the observational sparsity can be overcome physical features such as the interbasin gradients in deep 13cdic and the vertical structure of atlantic 13cdic are accurately reconstructed the state estimation method extracts useful information from the pointwise observations to infer distributions at the largest oceanic scales at least 10 000 km horizontally and 1500 m vertically and outperforms a standard optimal interpolation technique even though neither dynamical constraints nor constraints from surface boundary fluxes are used when the sparse observations are more realistically restricted to the paleoceanographic proxy observations of 13c 18o and cd ca however the large scale property distributions are no longer recovered coherently at least three more water mass tracers are likely needed at the core sites in order to accurately reconstruct the large scale property distributions of the last glacial maximum

reconstruction of sea surface conditions at middle to high latitudes of the northern hemisphere during the last glacial maximum lgm based on dinoflagellate cyst assemblages abstract a new calibration database of census counts of organic walled dinoflagellate cyst dinocyst assemblages has been developed from the analyses of surface sediment samples collected at middle to high latitudes of the northern hemisphere after standardisation of taxonomy and laboratory procedures the database comprises 940 reference data points from the north atlantic arctic and north pacific oceans and their adjacent seas including the mediterranean sea as well as epicontinental environments such as the estuary and gulf of st lawrence the bering sea and the hudson bay the relative abundance of taxa was analysed to describe the distribution of assemblages the best analogue technique was used for the reconstruction of last glacial maximum lgm sea surface temperature and salinity during summer and winter in addition to sea ice cover extent at sites from the north atlantic n 63 mediterranean sea n 1 and eastern north pacific n 1 three of the north atlantic cores from the continental margin of eastern canada revealed a barren lgm interval probably because of quasi permanent sea ice six other cores from the greenland and norwegian seas were excluded from the compilation because of too sparse assemblages and poor analogue situation at the remaining sites n 54 relatively close modern analogues were found for most lgm samples which allowed reconstructions the new lgm results are consistent with previous reconstructions based on dinocyst data which show much cooler conditions than at present along the continental margins of canada and europe but sharp gradients of increasing temperature offshore the results also suggest low salinity and larger than present contrasts in seasonal temperatures with colder winters and more extensive sea ice cover whereas relatively warm conditions may have prevailed offshore in summer from these data we hypothesise low thermal inertia in a shallow and low density surface water layer

ice sheet variability around the north atlantic ocean during the last deglaciation millennial scale variability in the flux of ice rafted detritus to north atlantic sediments during the last glacial period has been interpreted to reflect a climate forced increase in the discharge of icebergs from ice sheet margins surrounding the northern north atlantic ocean 1 but the relationship between ice sheet variability and climate change is not clear as both the sources of ice rafted detritus and the ice marginal processes are varied and complex 2 4 terrestrial records are helpful in unravelling this complexity because they can demonstrate the scale of ice sheet oscillations and whether the ice sheet or sector was advancing or retreating with respect to climate change here we constrain the age and anatomy of a prominent readvance of the british ice sheet in the northern irish sea region at similar to 14 14 c kyr bp similar to 16 4 calendar kyr bp the analysis indicates that the british ice sheet participated in an iceberg discharge episode known as heinrich event 1 comparison with other terrestrial and marine ice sheet records suggests that the dynamic collapse of the laurentide ice sheet beginning at 14 6 15 0 c 14 kyr bp1 4 similar to 17 2 17 6 calendar kyr bp 5 initiated varied responses from other ice sheet margins around the northern north atlantic region these observations support the argument that the release of icebergs and meltwater during heinrich event 1 disrupted the north atlantic thermohaline circulation 6 8 leading to a delay or reversal of deglaciation of the northern hemisphere and at least as far south as 40 degrees s for two to three thousand years 5 9 10 suggesting a climate forcing and response similar to that of the ensuing younger dryas cold snap 11 12

downcore variations of sedimentary detrital 238 u 232 th ratio implications on the use of 230 th xs and 231 pa xs to reconstruct sediment flux and ocean circulation excess 231 pa and 230 th 231 pa xs and 230 th xs can be used to reconstruct past oceanic sedimenta tion 230 th normalized flux and circulation changes 231 pa 230 th xs 0 hereafter pa th these quantities are determined by computing the detrital and authigenic contributions from bulk sediment measurement the method relies on the use of a chosen constant value of the detrital 238 u 232 th activity ratio hereafter u th det in this study we have extracted the detrital fraction of the sediments from north atlantic deep sea core su90 08 43803 0 1n 30802 0 5w 3 080 m and determined its u th det value over the last 40 ky we find that u th det varied significantly through time with a minimum value of 0 4 during the holocene and a maximum value of 0 7 during the last glacial maximum lgm the sensitivity of sedimentary 230 th normalized flux and pa th is tested for our study site and for other north atlantic sites we show that the sensitivity is highly dependent on the core location and its terrigenous material supply the 230 th normalized flux and pa th signals are very robust in cores with low detrital contributions whereas they are very sensitive to u th det changes in cores with higher detrital contribution in the latter case changes in 230 th normalized flux and pa th due to the choice of a constant u th det can largely exceed the uncertainty on the 230 th normalized flux and pa th inducing potential biases in the amplitude and temporal variability of reconstructed sedimentation and ocean circulation changes

a record of the last 460 thousand years of upper ocean stratification from the central walvis ridge south atlantic the upper branch of the atlantic meridional overturning circulation predominantly enters the atlantic ocean through the southeast where the subtropical gyre is exposed to the influence of the agulhas leakage al to understand how the transfer of indian ocean waters via the al affected the upper water column of this region we have generated new proxy records of planktic foraminifera from a core on the central walvis ridge on the eastern flank of the south atlantic gyre sag we analyzed the isotopic composition of subsurface dweller globigerinoides ruber sensu lato and thermocline globorotalia truncatulinoides sinistral spanning the last five pleistocene glacial interglacial g ig cycles the former displays a response to obliquity suggesting connection with high latitude forcing and a warming tendency during each glacial termination in response to the interhemispheric seesaw the 18o difference between the two species interpreted as a proxy for upper ocean stratification reveals a remarkably regular sawtooth pattern bound to g ig cyclicity it rises from interglacials until glacial terminations with fast subsequent decrease appearing to promptly respond to deglacial peaks of al stratification however bears a different structure during the last cycle being minimal at last glacial maximum and peaking at termination i we suggest this to be the result of the intensified glacial wind field over the sag and or of the invasion of the south atlantic thermocline by glacial north atlantic intermediate waters the 13c time series of the two species have similar g ig pattern whereas their difference is higher during interglacials we propose that this may be the result of the alternation of intermediate water masses in different circulation modes and of a regionally more efficient biological pump at times of high pco2

rapid transient changes in northeast atlantic deep water ventilation age across termination i 1 a sequence of accelerator mass spectrometry ams 14c dates performed on benthic and planktonic foraminifera from a northeast atlantic deep sea core md99 2334k 37 48 n 10 10 w 3146 m permit the reconstruction of deep water 14c ventilation ages across the last deglaciation the records from md99 2334k have been placed on the gisp2 timescale via the synchrony of temperature changes recorded in the greenland ice cores and in north atlantic planktonic 18occ calcite 18o on the basis of a range of estimates for past source water 14c this permits the estimation of 14c projection ventilation ages for comparison with benthic planktonic 14c age differences although the accurate estimation of past ventilation ages is precluded by unknown deep water 14c source signatures and by uncertainty regarding the extent of deep water mixing it is clear that deep water ventilation in the northeast atlantic was significantly reduced during the last glaciation increased abruptly coincident with the bolling allerod warming and rapidly became reduced again during the younger dryas cold reversal the character of these changes is consistent with a varying dominance of north atlantic deep water nadw versus antarctic bottom water aabw parallel benthic 13c deep water temperature tdw and deep water 18o 18odw estimates support this inference the fact that deglacial changes in the deep water radiocarbon content of the northeast atlantic run parallel to opposite changes in atmospheric radiocarbon content and in parallel with greenland temperature fluctuations unequivocally implicates changes in ocean circulation in deglacial climate evolution and illustrates the capacity for the deep ocean to respond and contribute to abrupt climate change

200 000 yr diatom records from atlantic upwelling sites reveal maximum productivity during lgm and a shift in phytoplankton community structure at 185 000 yr abstract time series of diatom accumulation rates dar and assemblage composition for the last 200 000 yr from the atlantic major coastal upwelling loci of the eastern boundary currents and eastern equator are compared together with independent proxies of export production to upper ocean productivity and to nutrient concentration and utilization fluctuations in dar are concomitant at all sites and in good correspondence to the other independent productivity proxies indicating glacial periods as the most productive times of the past 200 000 yr in both hemispheres of the eastern atlantic maximum productivity about one order of magnitude higher than at present occurred at the last glacial maximum lgm despite the atlantic similarities in both coastal and equatorial upwelling types important regional and site specific variations emerge in the southern hemisphere site a contradiction of proxies diatoms and corg is noted at the base of stage 6 185 000 yr the good correlation found between the corg and the benthic foraminifera fluxes is interpreted as a reflection of a shift in the phytoplankton community structure related to either a change in the n p si ratio or similar nutrient conditions associated with increased water column stratification

flow dynamics of an accumulation basin a case study of upper kahiltna glacier mount mckinley alaska we interpreted flow dynamics of the kahiltna pass basin accumulation zone on mount mckinley alaska usa using 40 100 and 900mhz ground penetrating radar profiles and gps surface velocity measurements we found dipping englacial surface conformable strata that experienced vertical thickening as the glacier flowed westward from a steep higher velocity 60ma 1 region into flat terrain associated with a 908bend in the glacier and lower velocities 15ma 1 to the south stratigraphy near the western side of the basin was surface conformable to 170m depth and thinned as flow diverged southward down glacier we found complex strata beneath the conformable stratigraphy and interpret these features as buried crevasses avalanche debris and deformed ice caused by up glacier events we also suggest that basin dimensions bed topography and the sharp bend each cause flow extension and compression significantly contributing to conformable and complex strata thickness variations our findings show that surface conformable stratigraphy continuous with depth and consistent strata thicknesses cannot be assumed in accumulation basins because local and up glacier terrain and flow dynamics can cause structural complexities to occur under and within surface conformable layers

230th normalization an essential tool for interpreting sedimentary fluxes during the late quaternary there is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates notwithstanding its common incidence this process generally referred to as sediment focusing is often difficult to recognize if redistribution is near synchronous to deposition the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked ignoring it however can lead to serious misinterpretations of sedimentary fluxes particularly when past changes in export flux from the overlying water are inferred in many instances this problem can be resolved at least for sediments deposited during the late quaternary by normalizing to the flux of 230th scavenged from seawater which is nearly constant and equivalent to the known rate of production of 230th from the decay of dissolved 234u we review the principle advantages and limitations of this method notwithstanding its limitations it is clear that 230th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates

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

compositional variation in the 1983 and other recent etnean lavas insights on the shallow feeding system a systematic petrographic and chemical study of chronologically ordered lava samples collected during the 1983 etna eruption has shown minor fluctuating variations in crystallinity and chemistry apparently correlated with variations in the rate of effusion comparison with the compositional variations observed in the lavas emitted during the 1971 1983 period has revealed a repeated evolutional tendency suggesting a small size shallow storage system periodically refilled as in 1983 by more primitive magma

two phase structure of tropical hydroclimate during heinrich stadial 1 and its global implications abstract previous studies have proposed that the two phase structure of heinrich stadial 1 hs1 is a ubiquitous feature however few studies have focused on the internal structure of hs1 over a tropical domain thus the interhemispheric ocean atmosphere teleconnections during hs1 are poorly defined here we present high resolution sedimentological geochemical and palaeoceanographic records from the core cg2 in the southern south china sea scs off the northern sunda shelf the core continuously spans 9 0 22 0 ky bp the records suggest two episodes of increased monsoon rainfall during the pre hs1 19 0 18 0 ky bp and in the early hs1 17 5 16 1 ky bp along with a weaker monsoon rainfall during the late hs1 16 1 14 7 ky bp in contrast a lower precipitation over the flores sea occurred during the early hs1 off the southern sunda shelf followed by an enhanced precipitation runoff during the late hs1 muller et al 2012 notably these two different precipitation phases characterise a southward intertropical convergence zone itcz shift over the sunda shelf during hs1 these changes are also consistent with other monsoon records as well as other available data from the tropical to high latitudes of both hemispheres supporting the global scope of the twofold hs1 we propose that the two episodes of intense monsoon rainfall over the northern sunda shelf during the pre hs1 and the early hs1 observed in our study are closely linked to two different melting events associated with european ice sheet eis retreat we further propose that the subsequent maximum atlantic meridional overturning circulation amoc reduction and the migration of the itcz to its southernmost position during the late hs1 are tightly connected with the destabilization of the marine terminating ice streams of the laurentide ice sheet lis in the north atlantic tropical positive feedbacks during hs1 in turn should have played an important role in sustaining the amoc reduction and northern hemisphere nh stadial conditions by continuously lowering the salinity of the low latitude currents in the atlantic ocean in addition the tropical itcz appears to have been a key bridge in promoting interhemispheric climate connections by which north atlantic cooling was rapidly linked to increased southern ocean surface westerlies this in turn drove upwelling and co2 ventilation from the southern ocean and thereby facilitated deglacial warming finally these findings show that tropical climate variation is an important component of deglacial climate changes understanding how it responds to millennial events like hs1 may help us better understand global teleconnections

phase relationships between millennial scale events 64 000 24 000 years ago a core recovered on the iberian margin off southern portugal can be correlated with greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over greenland our age model identifies the base of every interstadial between 64 000 and 24 000 years ago and uses the greenland ice core project grip timescale the oxygen isotope signal in benthic foraminifera on this grip based timescale is quite different from the planktonic record and resembles the temperature record over antarctica when this is synchronized with greenland using the record of methane in the atmospheric air in the polar ice cores we interpret the benthic record as indicating significant fluctuations in ice volume during millennial events and we suggest that antarctic temperature changed as a function of ice volume

anatomy of a dansgaard oeschger warming transition high resolution analysis of the north greenland ice core project ice core large and abrupt temperature oscillations during the last glacial period known as dansgaard oeschger do events are clearly observed in the greenland ice core record here we present a new high resolution chemical 2 mm and stable isotope 20 mm record from the north greenland ice core project ngrip ice core at the onset of one of the most prominent do events of the last glacial do 8 observed 38 000 years ago the unique subannual resolution ngrip record provides a true sequence of change during a do warming with detailed annual layer counting of very high depth resolution geochemical measurements used to determine the exact duration of the transition the continental ions indicative of long range atmospheric loading and dustiness from east asia are the first to change followed by the snow accumulation the moisture source conditions and finally the atmospheric temperature in greenland the sequence of events shows that atmospheric and oceanic source and circulation changes preceded the do warming by several years

millennial scale dynamics of the winter cold tongue in the southern south china sea over the past 26 ka and the east asian winter monsoon millennial scale variations of the east asian winter monsoon eawm remain elusive due to sparse and controversial reconstructions by compiling a variety of alkenone based sea surface temperature sst estimates we find that the west east sst gradient in the southern south china sea scs well documents the temporal dynamics of the winter cold tongue off the southern vietnam and by inference variations in the eawm intensity over the past 26 ka our results reveal that the winter cold tongue ssts were significantly colder during heinrich event 1 and the younger dryas event resulting in an increased west east sst gradient in the southern scs due to a strengthened eawm within dating uncertainties an intensified eawm during cold stadials was coeval with the shutdown or a reduction in strength of the atlantic meridional overturning circulation amoc exhibiting a strong linkage between the amoc and the eawm system the west east sst gradient also indicates an enhanced eawm during the early holocene which may be induced by postglacial ice sheet dynamics and a strong seasonal contrast in solar insolation our findings suggest that the eawm was probably modulated by a complex interplay between the amoc solar insolation and ice sheet dynamics on sub orbital time scales

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

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

circulation changes in the faeroe shetland channel correlating with cold events during the last glacial period 58 10 ka a core from the margin of the faeroe shetland channel covering the last glacial period 58 10 ka reveals a very detailed record of oscillations in benthic and planktic foraminifera oxygen isotopes and ice rafted detritus the oscillations correlate almost exactly with the dansgaard oeschger cycles in the greenland ice cores including similar subdivision of the cycles into warmer interstadial and colder stadial sections the remarkably close agreement between the benthic faunas and their 18 o data and the record of the ice cores provides new empirical evidence for a close relationship between the deep ocean circulation and the abrupt climatic changes of the last glacial period

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

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

southern ocean origin for the resumption of atlantic thermohaline circulation during deglaciation during the two most recent deglaciations the southern hemisphere warmed before greenland at the same time the northern atlantic ocean was exposed to meltwater discharge which is generally assumed to reduce the formation of north atlantic deep water yet during deglaciation the atlantic thermohaline circulation became more vigorous in the transition from a weak glacial to a strong interglacial mode here we use a three dimensional ocean circulation model to investigate the impact of southern ocean warming and the associated sea ice retreat on the atlantic thermohaline circulation we find that a gradual warming in the southern ocean during deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation triggered by increased mass transport into the atlantic ocean via the warm indian ocean and cold pacific ocean water route this effect prevails over the influence of meltwater discharge which would oppose a strengthening of the thermohaline circulation a southern ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of southern and northern hemispheric climate change at times of deglaciation in agreement with the available proxy records

sea surface temperature reconstructions over the last 70 kyr off portugal biomarker data and regional modeling this study aims at providing robust temperature reconstructions for a key oceanographic setting in the north atlantic and at understanding the nature of the temperature signal recorded by the two biomarkers u 37 k and tex86 considering season and depth of production to do so high resolution signals of u 37 k and tex86 are determined for the last 70kyr for core md95 2042 located off portugal signals of u 37 k and tex86 present a tight correlation demonstrating a dominant temperature effect u 37 k signals correspond well to the annual mean sea surface temperature sst whereas tex86h derived temperatures are 5 6 degrees c higher which is unrealistically warm for this area unsuitable tex86 global linear calibrations on the iberian margin may suggest a possible occurrence of archaeal communities with specific temperature response to assess the impact of different season or depth of production of the biomarkers on the recorded temperature in the sediment modeled temperature proxies tproxies are introduced in a regional oceanic modeling system and tested for different seasons annual summer winter and depths surface and 0 200m of production for three climate modes present day pd last glacial maximum lgm and heinrich stadials hs similar temperature amplitudes between climate modes are found at md95 2042 core site for observations for both biomarkers and for modeled annual surface production tproxy 5 5 7 degrees c for t pd lgm and 3 4 degrees c for t lgm hs therefore we propose a new tex86h regional calibration to reconstruct present and past annual mean ssts on the iberian margin

rare earth element association with foraminifera abstract neodymium isotopes are becoming widely used as a palaeoceanographic tool for reconstructing the source and flow direction of water masses a new method using planktonic foraminifera which have not been chemically cleaned has proven to be a promising means of avoiding contamination of the deep ocean palaeoceanographic signal by detrital material however the exact mechanism by which the nd isotope signal from bottom waters becomes associated with planktonic foraminifera the spatial distribution of rare earth element ree concentrations within the shell and the possible mobility of ree ions during changing redox conditions have not been fully investigated here we present ree concentration and nd isotope data from mixed species of planktonic foraminifera taken from plankton tows sediment traps and a sediment core from the nw atlantic we used multiple geochemical techniques to evaluate how where and when rees become associated with planktonic foraminifera as they settle through the water column reside at the surface and are buried in the sediment analyses of foraminifera shells from plankton tows and sediment traps between 200 and 2938 m water depth indicate that only 20 of their associated nd is biogenically incorporated into the calcite structure the remaining 80 is associated with authigenic metal oxides and organic matter which form in the water column and remain extraneous to the carbonate structure remineralisation of these organic and authigenic phases releases ions back into solution and creates new binding sites allowing the nd isotope ratio to undergo partial equilibration with the ambient seawater as the foraminifera fall through the water column analyses of fossil foraminifera shells from sediment cores show that their ree concentrations increase by up to 10 fold at the sediment water interface and acquire an isotopic signature of bottom water adsorption and complexation of ree3 ions between the inner layers of calcite contributes significantly to elevated ree concentrations in foraminifera the most likely source of ree ions at this stage of enrichment is from bottom waters and from the remineralisation of oxide phases which are in chemical equilibrium with the bottom waters as planktonic foraminifera are buried below the sediment water interface redox sensitive ion concentrations are adjusted within the shells depending on the pore water oxygen concentration the concentration of ions which are passively redox sensitive such as ree3 ions is also controlled to some extent by this process we infer that a the nd isotope signature of bottom water is preserved in planktonic foraminifera and b that it relies on the limited mobility of particle reactive ree3 ions aided in some environments by micron scale precipitation of mnco3 this study indicates that there may be sedimentary environments under which the bottom water nd isotope signature is not preserved by planktonic foraminifera tests to validate other core sites must be carried out before downcore records can be used to interpret palaeoceanographic changes

regional and global benthic δ18o stacks for the last glacial cycle although detailed age models exist for some marine sediment records of the last glacial cycle 0 150 ka age models for many cores rely on the stratigraphic correlation of benthic 18o which measures ice volume and deep ocean temperature change the large amount of data available for the last glacial cycle offers the opportunity to improve upon previous benthic 18o compilations such as the lr04 global stack not only are the age constraints for the lr04 stack now outdated but a single global alignment target neglects regional differences of several thousand years in the timing of benthic 18o change during glacial terminations here we present regional stacks that characterize mean benthic 18o change for 8 ocean regions and a volume weighted global stack of data from 263 cores age models for these stacks are based on radiocarbon data from 0 to 40 ka correlation to a layer counted greenland ice core from 40 to 56 ka and correlation to radiometrically dated speleothems from 56 to 150 ka the regional 18o stacks offer better stratigraphic alignment targets than the lr04 global stack and furthermore suggest that the lr04 stack is biased 1 2 kyr too young throughout the pleistocene finally we compare global and regional benthic 18o responses with sea level estimates for the last glacial cycle

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

constraints from paleotracer data on the north atlantic circulation during the last glacial maximum we investigate the extent to which the paleotracers 13c and 18o constrain the north atlantic ocean general circulation during the last glacial maximum lgm the distinction is made in this work between the depiction of water mass distributions and the inference of the oceanic circulation the 13c data indicate that water masses formed in the north atlantic and in the southern ocean were distributed differently during the lgm but the extent of the differences cannot be quantified because of uncertainties in the end member composition reasonable choices of values for the end members produce volumes of lower north atlantic deep water nadw significantly smaller than in the modern atlantic in agreement with previous studies to explore the range of circulations consistent with this water mass distribution a coarse resolution inverse model of the north atlantic is constrained with 13c and 18o paleodata the model shows that the glacial period tracer distribution is fully consistent with the circulation rates estimated for the modern ocean however the paleodata are also consistent with a circulation rate of one half the modern flux of nadw and indeed there are an infinite number of rates of deep water movement that are possible to remove this ambiguity the central need is for a data type capable of setting the overall rates of water movement i e a clock such as that provided by geostrophy or a well sampled radioactive tracer

intensification and variability of ocean thermohaline circulation through the last deglaciation neodymium isotope ratios in the authigenic ferromanganese oxide component in a southeastern atlantic core reveal a history of the global overturning circulation intensity through the last deglaciation from a minimum during the last glacial maximum lgm north atlantic deep water nadw began to strengthen between 18 and 17 kyr cal bp approximately 2000 3000 years before the bolling warming it exhibits a gradually increasing baseline intensity that plateaus in the early holocene tracks increasing northern hemisphere insolation and parallels atmospheric co2 concentration millennial scale fluctuations are superimposed on this baseline nadw increase corresponding to events in northern hemisphere climate records the millennial excursions show sharp increases associated with the pre bolling retreat of continental glaciers and the bolling warming itself a decrease with allerod cooling and a recovery through the younger dryas and early holocene the data confirm a close link between deep ocean circulation and north atlantic climate changes there is no clear linkage with meltwater pulses indicating that rapid meltwater additions did not trigger observable mode changes in nadw production however rapid changes in north atlantic sea ice cover show a strong relationship with the millennial perturbations of nadw flux to the deep southern ocean the data indicate that the baseline intensity of nadw reaching the cape basin is paced by northern hemisphere insolation and that north atlantic sea ice coverage acts as a major amplifying link between the oceans and the atmosphere d 2004 elsevier b v all rights reserved

millennial scale variability of deep water temperature and δ18odw indicating deep water source variations in the northeast atlantic 0 34 cal ka bp 1 paired measurements of mg ca and 18occ calcite 18o in benthic foraminifera from a deep sea core recovered on the iberian margin md99 2334k 37 48 n 10 10 w 3 146 m have been performed in parallel with planktonic 18occ analyses and counts of ice rafted debris ird the synchrony of temperature changes recorded in the greenland ice cores and in north atlantic planktonic 18occ allows the proxy records from md99 2334k to be placed confidently on the gisp2 time scale this correlation is further corroborated by ams 14c dates benthic mg ca measurements in md99 2334k permit the reconstruction of past deep water temperature tdw changes since 34 cal ka bp calendar kiloyears before present using these tdw estimates and parallel benthic 18occ measurements a record of deep water 18o 18odw has been calculated results indicate greatly reduced tdw in the deep northeast atlantic during the last glaciation until 15 cal ka bp when tdw warmed abruptly to near modern values in parallel with the onset of the bolling allerod interstadial subsequently tdw reverted to cold glacial values between 13 4 and 11 4 cal ka bp in parallel with the younger dryas cold reversal and the h0 ice rafting event similar millennial scale tdw changes also occurred during the last glaciation indeed throughout the last 34 cal ka millennial 18odw and tdw changes have remained well coupled and are linked with ird pulses coincident with heinrich events 3 2 1 and the younger dryas when transitions to lower tdw and 18odw conditions occurred in general millennial tdw and 18odw variations recorded in md99 2334k describe an alternation between colder low 18odw and warmer high 18odw conditions which suggests the changing local dominance of northern sourced north atlantic deep water nadw versus southern sourced antarctic bottom water aabw the observed similarity of the tdw and gisp2 18oice records would therefore suggest a common component of variability resulting from the coupling of nadw formation and greenland climate a link between greenland stadials and the incursion of cold low 18odw aabw in the deep northeast atlantic is thus implied which contributes to the relationship between greenland climate and the millennial benthic 18occ signal since 34 cal ka bp

the last interglacial ocean the final effort of the climap project was a study of the last interglaciation a time of minimum ice volume some 122 000 yr ago coincident with the substage 5e oxygen isotopic minimum based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean last interglacial sea surface temperatures sst were compared with those today there are small sst departures in the mid latitude north atlantic warmer and the gulf of mexico cooler the eastern boundary currents of the south atlantic and pacific oceans are marked by large sst anomalies in individual cores but their interpretations are precluded by no analog problems and by discordancies among estimates from different biotic groups in general the last interglacial ocean was not significantly different from the modern ocean the relative sequencing of ice decay versus oceanic warming on the stage 6 5 oxygen isotopic transition and of ice growth versus oceanic cooling on the stage 5e 5d transition was also studied in most of the southern hemisphere the oceanic response marked by the biotic census counts preceded led the global ice volume response marked by the oxygen isotope signal by several thousand years the reverse pattern is evident in the north atlantic ocean and the gulf of mexico where the oceanic response lagged that of global ice volume by several thousand years as a result the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years these regional lead lag relationships agree with those observed on other transitions and in long term phase relationships they cannot be explained simply as artifacts of bioturbational translations of the original signals

the mode and mechanism of the last deglaciation oceanic evidence changes in ocean temperature carbonate productivity and ice rafted detritus in the north atlantic suggest that half of the northern hemisphere ice volume at the last glacial maximum had disappeared by 13 000 yr b p despite the still extensive limits of the ice sheets this early thinning of the ice sheets occurred during a time when summer insolation values were slowly rising but when pollen evidence south of the ice margins indicates cold dry air masses we infer that this rapid early ice disintegration 16 000 13 000 yr b p was caused by oceanic mechanisms 1 rising sea level causing increased calving along ice margins 2 the chilling of the sea surface by icebergs and meltwater reducing moisture extraction by the atmosphere and transport to the ice sheets and 3 winter freezing of the low salinity meltwater layer suppressing local moisture extraction and the regional influx of moisture bearing storms from lower latitudes in winter and hence starving the ice sheets these oceanic feedback mechanisms were strongest from 16 000 to 13 000 yr b p and weaker but still active from that date until the end of deglaciation at 6000 yr b p

changes in the oceanic 13c 12c ratio during the last 140 000 years high latitude surface water records detailed records of the carbon and oxygen isotopic ratios of neogloboquadrina pachyderma are compared between nine high latitude sediment cores from the northern and southern hemispheres covering the last 140 000 yrs the strong analogies between the 13c records permit to define a 13c stratigraphic scale with three clear cut transitions simultaneous with the oxygen isotopic transitions 65 125 kyrs 54 65 kyrs and 21 13 kyrs the 13c records of n pachyderma in the high latitude cores which follow the changes in 13c of the surface water tco2 near areas of deep water formation present trends similar to the benthic foraminifera 13c records in cores v19 30 and m12 392 although amplitudes of the isotopic shifts are different this implies that a large part of the observed variations represents global changes in the carbon distribution between biosphere and ocean the 13c12c ratios of n pachyderma in the north atlantic cores display larger regional variations at 18 kyrs b p than at present to explain these differences we have plotted the 18 kyrs b p 13c values of n pachyderma from 17 cores distributed n of 40 n comparison with published surface water temperature distribution at 18 kyrs b p indicates that a strong divergent cyclonic cell centered approximatively 55 n and 15 w was active during most of the last ice age maximum this hydrology analogous to the present weddell sea explains the published evidences of bottom water formation if located on the northern flank of the gyre and the strong polar front on the southern flank probable location of intermediate water formation

removal of 230th and 231pa at ocean margins abstract uranium thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the panama basin and by in situ filtration of large volumes of seawater in the panama and guatemala basins concentrations of dissolved th and pa isotopes were determined by extraction onto mno 2 adsorbers placed in line behind the filters in the in situ pumping systems concentrations of dissolved 230 th and 231 pa in the panama and guatemala basins are lower than in the open ocean whereas dissolved 230 th 231 pa ratios are equal to or slightly greater than ratios in the open ocean particulate 230 th 231 pa ratios in the sediment trap samples ranged from 4 to 8 in contrast to ratios of 30 or more at the open ocean sites previously studied particles collected by filtration in the panama basin and nearest to the continental margin in the guatemala basin contained 230 th 231 pa ratios similar to the ratios in the sediment trap samples the ratios increased with distance away from the continent suspended particles near the margin show no preference for adsorption of th or pa and therefore must be chemically different from particles in the open ocean which show a strong preference for adsorption of th ocean margins as typified by the panama and guatemala basins are preferential sinks for 231 pa relative to 230 th furthermore the margins are sinks for 230 th and to a greater extent 231 pa transported by horizontal mixing from the open ocean

the faroe shetland gateway late quaternary water mass exchange between the nordic seas and the northeastern atlantic abstract thirteen piston and gravity cores from the faroe shetland area were investigated for their planktic and benthic foraminiferal and oxygen isotopic distributions eight time slices between 18 ka bp and the present were reconstructed to study variations in surface and deep water exchange between the se norwegian sea and the northeast atlantic ocean today a relatively strong northward flow of warm north atlantic surface water is counterbalanced by a southward outflow of newly convected cold bottom water the norwegian sea overflow water during the last glacial maximum at 18 ka bp both the surface and bottom flows were slow and the climate conditions were arctic the convection north of the faroe area was weak and unstable the first indication of the deglaciation is a decrease in the planktic oxygen isotope values discernible southwest of the faroe islands at 15 5 ka bp the deglaciation proceeded northeast and eastward synchronous with a gradual intensification of northward flowing warmer atlantic intermediate water along the sea bottom meltwater fluxes increased between 14 and 13 ka bp producing cold surface waters and the climatic cooling was extreme there was no southward overflow of cold bottom water during this time period and the exchange of water masses between the nordic seas and the north atlantic ocean was essentially reversed i e estuarine during the bolling interstadial at 12 5 ka bp northward flowing warm surface water was present to the east of the faroe shetland channel wedged below a tongue of polar water spreading from the northwest and reaching into the faroe shetland channel convection in the nordic seas and overflow of cold deep water started during the bolling interstadial the polar water spread more eastward and southward during the following cold spell the younger dryas around 10 3 ka bp the polar water was overlying the warmer but more saline atlantic water which flowed northward below the cold surface water the overflow of cold bottom water was supposedly only slightly weaker than during the bolling interstadial strong inflow of warm surface water took place during the early holocene at 9 5 ka bp and relatively dense cold water flowed southward along the bottom the rate of water mass exchange reached a maximum at 6 5 ka bp when both the inflow of warm atlantic surface water and the outflow of cold dense bottom water appear to have been stronger than today

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

numerical reconstructions of the eurasian ice sheet and climate during the late weichselian abstract geological investigations undertaken through the quaternary environments of the eurasian north programme established ice sheet limits for the eurasian arctic at the last glacial maximum lgm sedimentary records of palaeo ice streams and uplift information relating to ice sheet configuration and the pattern of deglaciation ice sheet numerical modelling was used to reconstruct a history of the eurasian ice sheet compatible with these geological datasets the result was a quantitative assessment of the time dependent behaviour of the ice sheet its mass balance and climate and predictions of glaciological products including sediments icebergs and meltwater at the lgm ice cover was continuous from scandinavia to the arctic ocean margin of the barents sea to the north and the kara sea to the east in the west along the continental margin between the norwegian channel and svalbard the ice sheet was characterised by fast flowing ice streams occupying bathymetric troughs which fed large volumes of sediment to the continental margin that were deposited as a series of trough mouth fans ice streams may also have been present in bathymetric troughs to the north between svalbard and franz josef land further east however the ice sheet was thinner across the kara sea the ice thickness was predicted to be less than 300 m while on severnaya zemlya the ice cover may have been thinner at the lgm than at present it is likely that the taymyr peninsula was mainly free of ice at the lgm in the south the ice margin was located close to the shoreline of the russian mainland the climate associated with this ice sheet is maritime to the west and in stark contrast desert like in the east atmospheric general circulation modelling has revealed that such a contrast is possible under relatively warm north atlantic conditions because a circulation system develops across the kara sea isolating it from the moisture laden westerlies which are diverted to the south ice sheet decay began through enhanced iceberg calving in the deepest regions of the barents sea which caused a significant ice embayment within the bear island trough by about 12 000 years ago further iceberg calving reduced ice extent to the northern archipelagos and their surrounding shallow seas ice decay was complete by about 10 000 years ago

radiometrically determined sedimentary fluxes in the sub polar north atlantic during the last 140 000 years abstract we have examined the record of sediment input during the last 140 000 years in a deep sea core from 49 27 n 22 16 w in the eastern north atlantic using uranium series disequilibria to constrain time we have calculated mass fluxes of total sediment as well as mass and particle fluxes of major sedimentary components sediment accumulation rates were generally lower and relatively constant during broadly defined interglacial intervals driven primarily by the burial of biogenic material accumulation rates were higher and more variable during glacial intervals and were influenced primarily by terrigenous material peaks in bulk mass fluxes were associated with particular episodes heinrich events within the last glacial and during each of the last two deglaciations the flux of ice rafted debris as uniquely identified by coarse detrital fragments was higher during layers representing each of the glacial heinrich events with modest increases during events h3 and h6 and dramatic increases during the others confirming the widespread interpretation of these layers as episodes of enhanced iceberg delivery the burial flux of foraminifera was markedly lower during each of the glacial heinrich events also confirming the original identification of these layers as barren intervals ice rafting events within marine isotope stage 5 left neither a large detrital nor biogenic flux imprint at our study site variations in the burial rates of non carbonate sediments were largely responsible for overall changes in sediment accumulation throughout the last climate cycle ice rafting was apparently an important delivery mechanism for this terrigenous material the instantaneous chronometer established here for the last 140 000 years in the subpolar north atlantic allows the transformation of existing and subsequent data from relative values to absolute burial fluxes

transferring radiometric dating of the last interglacial sea level high stand to marine and ice core records in order to derive a radiometric age marker for the end of the penultimate glacial interglacial transition we compiled published u series isotope measurements on corals from the period extending from stage 6 to the middle of the last interglacial and computed the corresponding open system ages using thompson et al model thompson w g spiegelman m w goldstein s l speed r c an open system model for u series age determinations of fossil corals earth planet sci lett 210 2003 365 381 we obtain a global mean age of 126 calendar kyr bp ka 1 7kyr 2 for the beginning of the last interglacial sea level high stand after showing that the phase relationships observed between changes in sea level north atlantic benthic and planktonic foraminifera oxygen isotopic records and atmospheric methane over the last deglaciation were likely also valid over the penultimate deglaciation we derive an age of 131 2ka 2kyr 2 for the abrupt increase in atmospheric ch4 and north atlantic surface temperature marking the end of the penultimate glacial interglacial transition this age is consistent with u th dates of the penultimate glacial interglacial transition recorded in speleothems from sites where speleothems isotopic records are synchronous with north atlantic temperature records over the last deglaciation finally we show that the phase obtained between the climatic response and northern hemisphere summer insolation is not constant from termination ii to termination i implying that northern hemisphere summer insolation alone cannot explain the timing of terminations

millennial scale iceberg discharges in the irminger basin during the last glacial period relationship with the heinrich events and environmental settings high resolution records of coarse lithic content and oxygen isotope have been obtained in a piston core from the irminger basin the last glacial period is characterized by numerous periods of increased iceberg discharges originating partly from iceland and corresponding to millennial scale instabilities of the coastal ice sheets and ice shelves in the nordic area a comparison with midlatitude sediment cores shows that ice rafted material corresponding to the heinrich events was deposited synchronously from 40 to 60 n there are thus two oscillating systems every 5 10 kyr massive iceberg armadas are released from large continental ice caps whereas more frequent instabilities of the coastal ice sheets in the high latitude regions occur every 1 2 3 8 kyr at the time of the heinrich events the synchroneity of the response from all the northern hemisphere ice sheets attests the existence of strong interactions between the two systems

enhanced scavenging of 231pa relative to 230th in the south atlantic south of the polar front implications for the use of the 231pa 230th ratio as a paleoproductivity proxy abstract the fractionation of 230th and 231pa was investigated throughout the atlantic sector of the southern ocean published scavenging models generally assume that the 231pa 230t ratio of surface sediments is primarily determined by the mass flux of particles this relationship holds north of the polar front where low primary productivity coincides with ratios of unsupported 231pa 230th xs 231pa 230th in surface sediments below the production ratio of both radionuclides in the water column however we observed high xs231pa 230th ratios conventionally interpreted as a high productivity signal in surface sediments south of the polar front especially throughout the weddell sea in contradiction with the low particle flux of this region measurements of both dissolved and particulate fractions of 231pa and 230th in the water column revealed a strong n s decrease in the th pa fractionation factor from typical open ocean values around 10 north of the polar front to values between 1 and 2 south of 60 s this observation clearly indicates that the high xs231pa 230th ratios in surface sediments south of the antarctic circumpolar current are produced by a n s increase in the relative scavenging efficiency of 231pa relative to 230th most probably due to a change in the chemical composition of particulate matter and not by a high mass flux it is speculated that biogenic opal suggested not to significantly fractionate231pa and 230th may explain the enhanced scavenging of 231pa to the south this assumption is further supported by extremely high 231pa 230th ratios up to 0 34 in material collected with sediment traps south of the polar front where fluxes are primarily determined by biogenic opal based on these results we conclude that in regions where the sedimenting flux is dominated by biogenic opal the 231pa 230th ratio is not a reliable indicator for the mass flux of particles thus limiting its use as a paleoproductivity proxy in the southern ocean

deep circulation of the north atlantic over the last 200 000 years geochemical evidence variations in the cadmium calcium ratio of north atlantic deep water are recorded in the fossil shells of benthic foraminifera the oceanic distribution of cadmium is similar to that of the nutrients hence the cadmium calcium ratio in shells records temporal variations in nutrient distributions data from a north atlantic sediment core show that over the past 200 000 years there has been a continuous supply of nutrient depleted waters into the deep north atlantic the intensity of this source relative to nutrient enriched southern waters diminished by about a factor of 2 during severe glaciations this evidence combined with carbon isotope data indicates that the continental carbon inventory may have been less variable than previously suggested

sudden changes in north atlantic circulation during the last deglaciation sudden changes in the flow of warm atlantic surface waters into the norwegian sea occurred frequently during the last deglaciation typically involving shifts in sea surface temperature of 5 c in fewer than 40 years these led to equally large and rapid changes in atmospheric temperatures and to shifts in atlantic deep thermo haline circulation and ice sheet melting rates

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

radiocarbon decay and oxygen utilization in the deep atlantic ocean a parameter based on the sum of the concentrations of po4 and o2 divided by the redfield coefficient o2 po4 is used to separate the contributions of the northern and southern components to deep waters in the atlantic this separation allows the amount of radiocarbon lost by radiodecay and the amount of oxygen lost to respiration during residence in the deep atlantic to be calculated maps of these quantities reveal strong west to east gradients and weak north to south gradients consistent with ventilation along the western boundary from both ends of the ocean coupled with mixing outward from the boundary the o2 and 14c deficiences are highly correlated suggesting an o2 utilization rate of 12 m kg per century the apparent mean isolation time of water in the deep atlantic is about 200 years

glacier flow in a curving channel the flow of a glacier along a channel of constant longitudinal curvature is analyzed using analytical and finite element methods channels of various cross sectional shape are investigated ranging from a simple rectangular form with zero shear traction along the bed to realistic profiles taken from blue glacier washington terms in the equilibrium and rate of deformation equations which are inversely dependent on radius and a body force which varies transversely across the glacier introduce several characteristic features into the stress and velocity fields of the curving glacier the stress center line is shifted toward the inside of the bend causing an asymmetric crevasse pattern and non zero stress magnitude at the surface on the geometric center line of the channel the stress field is dependent on the stress exponent in the flow law and is non linear across the surface the surface velocity pattern shows a tilting of the usual high order parabolic form being skewed toward the inside of the bend there is a shift in the velocity maximum from the deepest part of the channel all of these curvature induced features are dependent on the radius of curvature actual channel geometry and stress exponent in the flow law model results show excellent agreement with the velocity and crevasse patterns on the curving blue glacier

variations in terrigenous input into the deep equatorial atlantic during the past 24 000 years estimates of terrigenous fluxes at three different water depths at two sites in the equatorial atlantic by normalization against excess 230 th flux indicate that the flux of terrigenous material to the seafloor was significantly higher during the last glacial period than it is today fluxes started to decrease during deglaciation and reached minimal values in the middle of the holocene from 15 000 to 5 000 years ago there was a substantial increase in flux with increasing water depth below 2 800 meters this increase may reflect resuspension and lateral transport of slope and rise sediment possibly because of intensification of deepwater circulation during that period

the role of deep ocean circulation in setting glacial climates the glacial cycles of the pleistocene involve changes in the circulation of the deep ocean in important ways this review seeks to establish what were the robust patterns of deep sea water mass changes and how they might have influenced important parts of the last glacial cycle after a brief review of how tracers in the modern ocean can be used to understand the distribution of water masses i examine the data for biogeochemical circulation rate and conservative tracers during glacial climates some of the robust results from the literature of the last 30 years include a shoaled version of northern source deep water in the atlantic expanded southern source water in the abyss and deep ocean salt rather than heat stratification of the last glacial maximum lgm deep sea and several lines of evidence for slower overturning circulation in the southern deep cell we combine these observations into a new idea for how the ocean atmosphere system moves from interglacial to glacial periods across a single cycle by virtue of its influence on the melting of land based ice around antarctica cooling north atlantic deep water nadw leads to a cold and salty version of antarctic bottom water aabw this previously underappreciated feedback can lead to a more stratified deep ocean that operates as a more effective carbon trap than the modern helping to lower atmospheric co 2 and providing a mechanism for the deep ocean to synchronize the hemispheres in a positive feedback that drives the system to further cooling

ventilation of the deep northeastern atlantic comparison of the 1973 geosecs expedition results from the deep eastern basin of the north atlantic with those for 1981 tto expedition reveal no firm evidence for change in no3 po4 or a h4sio4 concentration while a 2 3 mol kg difference is seen for o2 it is more likely experimental than temporal in origin the combined tto geosecs data sets reveal no evidence for ventilation of the bottom waters of the eastern basin by waters from the north

a 190 drop in atmosphere s δ14c during the mystery interval 17 5 to 14 5 kyr reconstructions of atmospheric 14c for the deglacial period reveal a 190 10 drop between 17 5 and 14 5 kyr before present during the mystery interval while it appears that the major contributor must be the mixing between an isolated low radiocarbon abyssal reservoir and the remainder of the ocean a combination of 14c measurements on coexisting benthic and planktic foraminifera shells and 13c 12c measurements on benthic foraminifera make it difficult to identify a sufficiently large volume for this reservoir an alternative scenario based on higher 14c production rates between about 38 and 18 kyr ago would escape the need for a large isolated reservoir prior to the mystery interval but instead would call for a better ventilated ocean afterwards the addition of 14c deficient sedimentary carbon appears not to be an important contributor while the unlikely explanation that the half life of 14c is 15 greater than the accepted value 5730 yr would eliminate the steady decrease in the 14c c over the last 40 kyr it would not explain the steep drop which occurred between 17 5 and 14 5 kyr

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

structure of the upper water column in the northwest north atlantic modern versus last glacial maximum conditions 1 during the last glacial maximum the northwestern north atlantic constituted a major conduit for labrador and greenland ice sheet meltwaters vertical density gradients in its upper water masses have been reconstructed by combining information from transfer functions based on dinocysts and from oxygen isotope measurements 18o in planktonic foraminifera transfer functions yield temperature and salinity and thus potential density for the warmest august and coldest february months in the photic zone the 18o values in different size fractions of epipelagic globigerina bulloides and mesopelagic neogloboquadrina pachyderma left coiled npl foraminifera allow us to assess gradients through the pycnocline between surface and intermediate waters based on the calibration of a versus 18o relationship from transfer function reconstructions the size and density of npl shells provide further constraints on these gradients the results show the development of a very strong pycnocline during the lgm with a difference of about 3 summer to 1 5 winter units between surface and underlying waters they indicate conditions unfavorable for vertical convection and support the hypothesis of the spreading of a shallow low salinity buoyant layer over the northern north atlantic this layer depicted a strong e w gradient with maximum seasonal contrast and minimum absolute values westward

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments

promotion of meridional overturning by mediterranean derived salt during the last deglaciation we demonstrate that changes in the behavior of the mediterranean outflow water mow prior to and through the last deglaciation played an important role in promoting meridional overturning circulation moc estimation of past mow salt and heat fluxes indicates that they gradually increased through the last deglaciation between 17 5 and 14 6 thousand years ago ka b p where b p references year 1950 net evaporation from the mediterranean exported sufficient fresh water from the north atlantic catchment to cause an average salinity increase of 0 5 psu throughout the upper 2000 m of the entire north atlantic to the north of 25

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

quaternary oceans and climate change lessons for the future there is much interest in ice age studies in recent decades in the context of global warming the relevant findings are these large regular changes in climate occurred within the last million years especially in the northern north atlantic extreme conditions were similar suggesting strong negative feedback at the edges of the range of variation the nature of the periods of climate variation suggests orbital forcing by modulation of internal oscillations involving lagged negative feedback on ice buildup transitions from cold to warm were rapid and they were not readily reversed indicating that ice dynamics underlies abrupt climate change accelerated rates of ice decay upon warming correspond to a sea level rise of one to two m century millennial scale abrupt disturbances known as dansgaard oeschger and heinrich events occur when large ice masses are present in the northern hemisphere they may be considered experiments on the ocean s response to massive meltwater input when using results from ice age studies to project future developments one must be aware that the future will be largely outside of experience with regard to the recent geologic past also there are as yet no generally accepted explanations for striking changes in the past such as the rapid rise of carbon dioxide during deglaciation demonstrating a profound lack of understanding of climate dynamics this is so far the lesson from ice age studies they say much about the deficiencies in our level of understanding and not so much about what is ahead