origin of the northern atlantic s heinrich events as first noted by heinrich 1988 glacial age sediments in the eastern part of the northern atlantic contain layers with unusually high ratios of ice rafted lithic fragments to foraminifera shells he estimated that these layers are spaced at intervals of roughly 10 000 years in this paper we present detailed information documenting the existence of the upper five of these layers in odp core 609 from 50 n and 24 w their ages are respectively 15 000 radiocarbon years 20 000 radiocarbon years 27 000 radiocarbon years about 40 000 years and about 50 000 years we also note that the high lithic fragment to foram ratio is the result of a near absence of shells in these layers although we are not of one mind regarding the origin of these layers we lean toward an explanation that the heinrich layers are debris released during the melting of massive influxes of icebergs into the northern atlantic these sudden inputs may be the result of surges along the eastern margin of the laurentide ice sheet

carbon isotopic changes in benthic foraminifera from the western south atlantic reconstruction of glacial abyssal circulation patterns oxygen and carbon isotopic analyses have been performed on the benthic foraminifer planulina wuellerstorfi in seven late quaternary cores from the vema channel rio grande rise region the cores are distributed over the water depth interval of 2340 to 3939 m which includes the present transition from north atlantic deep water nadw to antarctic bottom water aabw the carbon isotopic records in the cores vary as a function of water depth the shallowest and deepest cores show no significant glacial interglacial difference in 13c four of the five cores presently located in the nadw have benthic foraminiferal 13c that is lower during glacial isotopic stages based on bathymetric gradients in 13c we conclude that like today there were two water masses present in the vema channel during glacial intervals a water mass enriched in 13c overlying another water mass depleted in 13c the largest gradient of change of 13c with depth however occurred at 2 7 km 1 km shallower than the present position of this gradient on the basis of paleontologic and sedimentologic evidence we consider it unlikely that the nadw aabw transition shallowed to this level reduced carbon isotopic gradients between the deep basins of the north atlantic and pacific oceans during the last glaciation suggest that production of nadw was reduced lower production of nadw may have modified the local abyssal circulation pattern in the vema channel region

the hierarchical structure of glacial climatic oscillations interactions between ice sheet dynamics and climate abrupt climatic oscillations around the north atlantic have been identified recently in greenland ice cores as well as in north atlantic marine sediment cores the good correlation between the dansgaard oeschger events in the ice and the heinrich events in the ocean suggests that climate in the north atlantic region underwent several massive reorganizations in the last glacial period a characteristic feature of these events seems to be their hierarchical structure every 7 to 10 thousand years when the temperature is close to its minimum the ice sheet undergoes a massive iceberg discharge this heinrich event is then followed by an abrupt warming then by several other oscillations each one lasting between one and two thousand years these secondary oscillations do not have a clear signature in marine sediments but constitute most of the dansgaard oeschger events found in the ice here we use a simplified model coupling an ice sheet and an ocean basin in order to illustrate how the interactions between these two components can lead to such a hierarchical structure the ice sheet model exhibits internal oscillations composed of ice sheet growing phases and basal ice melting phases that induce massive iceberg discharges these massive fresh water inputs in the ocean stop for a moment the thermohaline circulation enhancing the temperature contrast between low and high latitudes just after this event the thermohaline circulation restarts and an abrupt warming of high latitude regions is observed for some parameter values these warmer temperatures have in turn some influence on the ice sheet inducing secondary oscillations similar to those found in paleoclimatic records although the mechanism presented here may be too grossly simplified it nevertheless underlines the potential importance of the coupling between ice sheet dynamics and oceanic thermohaline circulation on the structure of the climatic records during the last glacial period

surface water temperature salinity and density changes in the northeast atlantic during the last 45 000 years heinrich events deep water formation and climatic rebounds we developed a new method to calculate sea surface salinities sss and densities ssd from planktonic foraminiferal 18o and sea surface temperatures sst as determined from planktonic foraminiferal species abundances sst sss and ssd records were calculated for the last 45 000 years for biogeochemical oceanic flux study bofs cores 5k and 8k recovered from the northeast atlantic the strongest feature is the dramatic drop in all three parameters during the heinrich ice rafting events we modelled the possibility of deepwater formation in die northeast atlantic from the ssd records by assuming that the surface waters at our sites cooled as they flowed further north comparison with modelled north atlantic deepwater densities indicates that there could have been periods of deepwater formation between 45 000 and 30 000 14c years b p interrupted by iceberg meltwater input of heinrich event 3 and 4 at 27 000 and 38 000 14c years b p and during the holocene no amount of cooling in the northeast atlantic between 30 000 and 13 000 years could cause deep water to form because of the low salinities resulting from the high meltwater inputs from icebergs our records indicate that after each heinrich event there were periods of climatic rebound with milder conditions persisting for up to 2000 years as indicated by the presence of warmer and more saline water masses after these warm periods conditions returned to average glacial levels these short term cold and warm episodes in the northeast atlantic are superimposed on the general trend towards colder conditions of the last glacial maximum lgm heinrich event 1 appears to be unique as it occurs as insolation rose and was coeval with the initial melting of the fennoscandian ice sheet we propose that meltwater input of heinrich event 1 significantly reduced north atlantic deep water formation reducing the heat exchange between the low and high latitudes thus delaying deglaciation by about 1500 radiocarbon years 2000 calendar years

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

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

primary production in the glacial north atlantic and north pacific oceans the conditions controlling primary production are very different in the modern north atlantic and north pacific oceans1 a difference that is reflected in the composition of diatom fossils in surface sediments by contrast i report here evidence that during the last glacial interval the diatom assemblage and by extrapolation the primary production was very similar in the two regions the modern analogues of these assemblages occur in sediments of baffin bay and the sea of okhotsk both highly productive seas where ice is present i infer that during the last glacial interval plankton biomass was at least as high as it is today in the north atlantic and was as much as an order of magnitude higher in the north pacific the glacial assemblage occurs in lithologies dominated by ice rafted detritus which is generally believed to indicate the presence of icebergs2 6 i hypothesize that the presence of numerous icebergs possibly associated with sea ice supported high production by physical mechanisms such as turbulent mixing and enhanced density stratification and or biogeochemical ones such as supply of major or trace nutrients

ocean chemistry during glacial time abstract measurements of co2 to air ratios in the gas trapped in bubbles in ice of glacial age suggest that the co2 content of the atmosphere was considerably lower during peak glacial time than during holocene time the purpose of this paper is to show that such a change must in all likelihood be the result of alterations in the nutrient element chemistry of sea water two possible scenarios are presented one involves alternate storage and erosion of phosphorous leaving residues from shelf sediments the other involves changes in the c p ratio in the organic debris falling to the deep sea means of verifying the nutrient cycle hypothesis are also given it is shown that the 13c record as we know it in planktonic and benthic foraminifera the oxygen record as inferred from benthic foraminifera species distributions and the early post glacial caco3 preservation event as recorded by aragonitic pteropods are consistent with both of the hypotheses presented only if an early post glacial spike in the 13c record for planktonic shells could be found would it be possible to eliminate one of these hypotheses i e that involving shelf storage the implications of these nutrient hypotheses to climate theory are as follows if shelf storage is responsible for the glacial to interglacial co2 increase then the co2 change must be considered an amplifier of some primary cause the reason is that sea level changes are needed to drive deposition on and erosion from the shelves on the other hand if changes in the c p ratio for falling debris are responsible then the co2 change could either be an amplifier or a primary cause for the major glacial to interglacial climatic cycle the latter is possible as self sustained oscillations in ocean chemistry might be driven by interactions between ocean ecology and ocean nutrient chemistry

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

evidence for two step deglaciation and its impact on north atlantic deep water circulation oxygen and carbon isotope records from benthic and planktonic foraminifera are presented for the past 35 000 years in the northeastern atlantic the results support the idea that the last deglaci ation took place in two major steps1 4 and conflict with theories calling for a strong reduction in north atlantic deep water formation to explain the abrupt cooling of the younger dryas cold period5 7

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

a model of oxygen isotope composition of precipitation implications for paleoclimate data the ratio of oxygen isotopes in glacial ice is often used as an indicator of climatic conditions during the period of time in which the ice was deposited we have developed a model which predicts the isotopic composition of snow given the composition of the source water and the meteorological path between the formation of an air water parcel in the boundary layer and the final deposition of precipitation onto the ice sheets after comparing the model results with isotope data from the present we draw inferences about the past isotopic composition of continental ice sheets our results depend strongly on the details of the meteorological path they are consistent with standard interpretations of isotope data but in some cases they allow alternate interpretations

cold outflow from the faroe bank channel abstract the exchange of water between the norwegian sea and the north atlantic around the faroe islands was investigated in 1987 and 1988 combining ctd data with shipborne acoustic doppler current profiler adcp measurements tests were devised to assess the accuracy of the adcp measurements and the results are described it is found that estimating a reference velocity for geostrophic measurements is limited by the presence of tides of order 10 cm s 1 in the outflow of cold water from the faroe bank channel which reaches speeds of nearly 1 m s 1 the error is acceptably small but strong time dependent components are revealed the discharge of cold water is traced 75 km beyond the sill of this channel and changes in potential temperature salinity and potential vorticity on an isopycnal surface are attributed to intense mixing an effective diffusivity near 100 cm2 s 1 is derived and k h instability demonstrated as a plausible source of the turbulent energy six current meter moorings were deployed from

deep circulation change linked to heinrich event 1 and younger dryas in a middepth north atlantic core a core from the mid atlantic ridge at 43 5 n and 3 km water depth shows distinct evidence of the deglacial events known as heinrich event 1 probably the marine equivalent of oldest dryas cooling in europe and the younger dryas the heinrich event dated at three levels to between 14 3 and 15 0 ka is marked by a minimum in foraminifera per gram by maxima in rates of sedimentation ice rafted debris per gram and relative abundance of n pachyderma s and by a 18o minimum in planktonic foraminifera the younger dryas event is marked by peak abundance of n pachyderma s and a planktonic 18o maximum benthic foraminiferal 13c reaches minimum values during both the heinrich event and the younger dryas our data indicate pronounced changes in surface water properties were coupled with reduced production of north atlantic deep water at each of these times

13c record of benthic foraminifera in the last interglacial ocean implications for the carbon cycle and the global deep water circulation the 13c 12c ratios of upper holocene benthic foraminiferal tests genera cibicides and uvigerina of deep sea cores from the various world ocean basins have been compared with those of the modern total carbon dioxide tco2 measured during the geosecs program the 13c difference between benthic foraminifera and tco2 is 0 07 0 04 for cibicides and 0 83 0 07 for uvigerina at the 95 confidence level 13c analyses of the benthic foraminifera that lived during the last interglaciation isotopic substage 5e about 120 000 yr ago show that the bulk of the tco2 in the world ocean had a 13c value 0 15 0 12 lower than the modern one at the 95 confidence level reflecting a depletion compared to the present value of the global organic carbon reservoir regional differences in 13c between the various oceanic basins are explained by a pattern of deep water circulation different from the modern one the antarctic bottom water production was higher than today during the last interglaciation but the eastward transport in the circumpolar deep water was lower

changes in the vertical structure of the north atlantic ocean between glacial and modern times the distribution of planktic foraminiferal 18o over the north atlantic ocean has been interpreted by duplessy et al 1991 1992 in terms of the distribution of surface water salinity during the last glacial maximum lgm and the younger dryas yd we present here the implications of this surface salinity distribution for changes in deep convection during these periods temperature salinity density diagrams of the water column have been constructed using the plancktic and benthic foraminifera 18o values together with sea surface temperature estimates obtained by using micropaleontological transfer functions the precision of these reconstructions is limited by the remaining uncertainties in the two basic assumptions upon which the methods are based the sea surface temperature reconstruction by climap are valid within the statistical uncertainties the water salinity 18o relationship is well constrained for surface and intermediate ocean waters a significant difference between the modern and the reconstructed glacial water structure is observed during the lgm the modern relatively warm and salty norwegian sea surface water and the north atlantic deep water nadw were replaced by low salinity surface water in the northern atlantic ocean and the norwegian sea and cold deep water 0 1 c however surface water salinity in the central north atlantic ocean 52 54 n 25 40 w was sufficiently high to permit deep water convection as a source of north atlantic intermediate and deep waters open ocean convection at the northern limit of the subtropical surface waters around 40 n may have contributed to the ventilation of the intermediate waters the waters deeper than 2000 m were the result of mixing between poorly oxygenated southern waters and well ventilated north atlantic deep waters a similar reconstruction for the cold yd period indicates that the modern type of circulation with warm and ventilated nadw was largely in operation and that the hydrologic pattern was different from that prevailing during the lgm

ice rafted evidence of long term north atlantic circulation abstract the absolute and relative rates of deposition of ice rafted volcanic and terrigenous clastic material were determined for the interval from the base of oxygen isotope stage 5 127 000 yrs b p to the lower part of stage 3 50 000 yrs b p in five subpolar north atlantic deep sea cores during this time interval global climate changed from a full interglaciation to nearly a full glaciation for the sediment derived from the continents dominantly quartz and feldspar the eastward decrease in deposition rates during interglacial intervals and eastward increase during glacial intervals is best explained by a southeastward jump in the locus of iceberg melting during climates colder than that of today a similar model may explain the eastward increase in volcanogenic debris during glacial time intervals however the eastward increase of volcanogenic debris during interglacial stage 5 requires a stronger southeastward component of flow from greenland than is evident in the modern circulation

patterns of ice rafted detritus in the glacial north atlantic 40 55 n the observation by heinrich 1988 that during the last glacial period much of the input of ice rafted detritus to the north atlantic sediments may have occurred as a succession of catastrophic events rekindled interest on the history of the northern ice sheets over the last glacial period in this paper we present a rapid method to study the distribution of these events both in space and time using whole core low field magnetic susceptibility we report on approximately 20 cores covering the last 150 to 250 kyr well defined patterns of ice rafted detritus appear during periods of large continental ice sheet extent although these are not always associated within their maxima most of the events may be traced across the north atlantic ocean for the six most recent heinrich layers hl two distinct patterns exist hl1 hl2 hl4 hl5 are distributed along the northern boundary of the glacial polar front over most of the north atlantic between 40 and 50 n hl3 is more restricted to the central and eastern part of the northern atlantic the nd sr isotopic composition of the material constituting different heinrich events indicates the different provenance of the two patterns hl3 has a typical scandinavia arctic icelandic young crust signature and the others have a large component of northern quebec and northern west greenland old crust material these isotopic results obtained on core su 9008 from the north american basin are in agreement with the study by jantschik and huon 1992 who used k ar dating of silt and clay size fractions of an eastern basin core me 68 89 these data confirm the large spatial scale of these events and the enormous amount of ice rafted detritus they represent

suborbital timescale variability of north atlantic deep water during the past 200 000 years we generated 200 kyr long proxy records of surface and deepwater variability from a subpolar north atlantic core v29 202 enabling us to assess the linkage between surface and deepwater changes on suborbital timescales in particular we used a benthic 13c record to evaluate the deep water response to dansgaard oeschger temperature oscillations and to heinrich events times of massive iceberg delivery to the north atlantic we found that the reduction of north atlantic deep water nadw production was generally associated with cold or dropping sea surface temperatures ssts as indicated by planktonic foraminiferal assemblages the nadw contribution to the site did not drop appreciably during heinrich events h4 through h2 probably because these events followed intervals of prolonged surface cooling already characterized by low rates of nadw production by contrast nadw reduction appears to have been synchronous with h5 sst rise associated with both dansgaard oeschger oscillations and heinrich events was usually accompanied by increasing nadw strength in a few cases the nadw recovery appeared to lag the sst rise however the apparent delay is most likely an artefact of the sedimentary record low concentrations of benthic foraminifera as a result of low benthic foraminiferal abundances during stage 6 the stage 6 benthic foraminiferal 13c record is of lower resolution than the younger part of the record the stage 6 proxy records for surface hydrography nevertheless reveal millennial scale oscillations similar to those seen in stage 3 the available 13c data suggest that nadw weakened in association with the cold portions of stage 6 sst oscillations we also sought to confirm a recent study which concluded that there was little nadw variability during the peak of the last interglaciation marine oxygen isotope substage 5e eemian isotope stage 5 was marked by a trend of increasing benthic 13c in v29 202 rising 13c through isotope stage 5 is also seen in tropical surface water records and at some deep atlantic sites and may reflect the common derivation of these water masses variations of 0 5 superimposed on this rising 13c trend within substage 5e in v29 202 are so far not evident in tropical feed waters and may therefore indicate that nadw production was weaker during the late than mid eemian an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type quit to leave the system paper 95pa02089 suborbital timescale variability of north atlantic deep water during the past 200 000 years by d w oppo and s j lehman diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

sequence of thrusting and syntectonic sedimentation in the eastern sub atlas thrust belt dades and mgoun valleys morocco a series of balanced cross sections across the sub atlas thrust belt and the northern ouarzazate basin are used to illustrate the structural geometry and the timing of deformation at the southern front of the high atlas mountains of morocco the selected area is among the best sedimentary records of mountain building of the entire orogenic system the study of the relationships between thrusts and synorogenic continental formations enables the unraveling of kinematic sequences and the proposal of a relative chronology of deformation active thrusting in the area occurred in a rather continuous fashion from the oligocene to the pliocene punctuated by a major erosional phase imprecisely placed in late oligocene to early miocene times detrital sedimentary facies indicate that uplift in the hinterland of the high atlas to the north of the sub atlas belt was taking place already by mid eocene times although it might have commenced locally even earlier within the sub atlas zone the exposed faults did not propagate in a simple piggy back fashion but show evidence of a complex synchronous sequence with events of fault reactivation and out of sequence thrusting

role of the thermohaline circulation in the abrupt warming after heinrich events evidence of rapid climate oscillations during the last glacial period has been identified in climate records from greenland ice cores1 2 and ocean sediments in the north atlantic3 4 these records show that periods of gradual cooling are terminated by abrupt warming events5 with the coldest periods coinciding with the deposition of ice rafted debris so called heinrich events throughout the north atlantic heinrich events are thought to be a signature of massive iceberg discharges owing to collapse of the laurentide ice sheet bond et al 5 have proposed that the decrease in meltwater flux following collapse and retreat of the ice sheet enhanced the ocean s thermohaline circulation6 thereby increasing advection of heat from the tropics and giving rise to abrupt climate warming here we test this idea using a simple ocean model coupled to a model of a periodically surging ice sheet we find that massive discharges of icebergs first stop the thermohaline circulation because of the consequent freshwater influx cooling the north atlantic region this is followed by a rapid restart of the circulation leading to abrupt warming thus our model can reproduce the qualitative features of the climate oscillations seen in the ice core and ocean records

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

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

variability in the deep and intermediate water circulation of the atlantic ocean during the past 25 000 years northern hemisphere modulation of the southern ocean the relative flux of north atlantic deep water nadw out of the atlantic can be monitored in the southern ocean where high 13c nadw mixes rapidly with low 13c recirculated pacific water before forming antarctic bottom water aabw by using 13c measured in benthic foraminifera as a water mass tracer it is shown that the southern ocean was dominated by pacific water with little nadw contribution during the last glaciation these tracer results document the direct physical connection between insolation controlled northern hemisphere oscillations and changes in the chemical and physical properties of southern ocean water the 13c records from the deep tropical atlantic document the transition from an atlantic dominated by southern ocean water during the last glaciation to an atlantic dominated by nadw during the holocene with more southern ocean water in the deep atlantic during the last glaciation the boundary between nadw and southern ocean water was north of its modern position allowing more southern ocean water to enter the eastern atlantic through the romanche fracture zone carbon isotope records from the caribbean and mediterranean seas suggest that the influence of mediterranean overflow water mow extended to the caribbean during the last glaciation and therefore was a volumetrically much more important water mass in the intermediate depth atlantic than it is today

the subpolar mode water of the north atlantic ocean abstract the warm waters of the subtropical and subpolar basins of the north atlantic have tight regional temperature salinity relationships and are conventionally called the regional central waters a volumetric census of the temperature salinity characteristics of the north atlantic by wright and worthington 1970 shows that waters characterized by certain segments of the t s relationships have large volumes compared to those of other segments volumetric mode waters such mode waters appear as layers with increased vertical separation between isopycnals pycnostads the present study reports on the existence of pycnostads in the central and eastern north atlantic these subpolar mode waters are formed by deep winter convection in the subpolar north atlantic and participate in the upper water circulation of the northern north atlantic the seasonal outcropping of the pycnostads occurs within and adjacent to the north atlantic current the irminger current the east and west greenland currents and

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

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

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

high resolution stratigraphy of the faeroe shetland channel and its relation to north atlantic paleoceanography the last 87 kyr abstract two piston cores from the faeroe shetland channel have been investigated for stratigraphic and paleoceanographic correlation with atlantic ocean and norwegian sea records the high resolution cores have been analyzed for the downcore distribution of planktic and benthic foraminifera and oxygen isotopes ams 14 c dates and the position of ash layers i and ii form the basis for the correlation core na81 10 covers the period from isotope stage 3 to recent stage 1 while core na81 04 covers isotope stages 5a to 1 the holocene is well represented in core na81 10 by 2 m of sediment whereas it is only 50 cm in core na81 04 which is situated directly under the main core of the norwegian sea deep water outflow the transition of isotope stage 2 to stage 1 is affected by dissolution and thus is less clearly represented ams 14 c dates show that termination ia begins at 16 ka and ends at about the 13 ka level the last glacial maximum lgm defined at 19 16 ka is characterized by an ird peak faunal planktic foraminifera maxima dated at 17 3 and 18 25 ka and a high percentage of globigerina quinqueloba apparently open water conditions and circulation within the norwegian sea prevailed at least from 22 5 to 14 8 ka during the greatest part of stage 2 including the lgm and well onto termination ia a preservation maximum is observed throughout stage 2 isotope stage 3 with a nearly complete record of isotope substages is well represented in both cores oxygen isotope depletion peaks correlate with fauna minima and in time to north atlantic heinrich events h5 h1 faunal data indicate that deep water is generated within the norwegian greenland sea repeatedly since 26 ka with stops prior to 26 ka 22 5 20 8 20 5 ka and between 14 9 and 13 ka h3 h2 and h1 all isotope depletion peaks since ca 50 ka are anticorrelated with fauna and are mainly controlled by surface salinity changes isotope stages and substages are directly correlated to fauna peaks and temperature between 87 and ca 50 ka the faeroe shetland channel record shows that repeated presences of atlantic surface water and deep convection alternating with convection shutdowns have occurred within the norwegian sea greenland sea from the beginning of stages 5a to 1

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

changes in sea surface hydrology associated with heinrich event 4 in the north atlantic ocean between 40 and 60 n abstract the changes in distribution of sea surface temperature and salinity in the north atlantic between 40 and 60 n were reconstructed for the time interval between 40 and 30 kyr bp which includes the large iceberg discharge event associated with the deposition of heinrich layer 4 we found that the meltwater input during deposition of heinrich layer 4 resulted in a 1 2 kyr temperature decrease of about 2 c and a salinity decrease in the range of 1 5 3 5 between 40 and 50 n sites above 50 n did not experience significant salinity variations a much larger area was affected by the reduction in sea surface temperature the amplitude of the sea surface temperature shift was however much smaller than the atmospheric temperature changes over greenland at gisp and grip sites