identification et distribution des grandes masses d eau dans les mers du labrador et d irminger the main deep water masses present at the time of the css hudson cruises in labrador and irminger seas in june 1990 and october november 1991 have been identified using characteristic temperatures t and salinities s the purpose of this study was to establish the transfer functions between micropaleontological assemblages of top sediments and thermohaline characteristics of water masses the water mass at the top of the labrador sea labrador sea water lsw is formed after intense movements of winter convection in the first 900 m depth of the water column below that depth the lsw parameters reach a double minimum s 34 80 and t 2 9 c only the sediments located on the continental slopes of greenland and labrador between depths of 500 and 1500 m are in contact with the lsw below the lsw the superior fraction of the north east atlantic deep water neadw1 is characterized by a temperature maximum 3 3 c and as such is distinguishable from the inferior fraction neadw2 the latter is ch

sr and nd isotopes as tracers of north atlantic lithic particles paleoclimatic implications sr and nd isotopic compositions have been measured on the lithic fraction of last climatic cycle sediments from the north atlantic 40 n 60 n in order to identify the origins of the particles from the reconstruction of their transport pathways we deduce the mechanisms that explain their distributions the main source regions are the canadian shield mostly the area of baffin bay and western greenland the scandinavian shield the european region british isles and bay of biscay and iceland we observe a significant glacial interglacial contrast characterized by a dominant icelandic input via near bottom transport by north atlantic deep water nadw during the interglacials and a largely continent derived contribution of surface transported ice rafted detritus ird during the glacial period during the last glacial period the heinrich events abrupt massive discharges of ird originated not only from the laurentide ice sheet as heretofore envisioned but also from other sources three other major north atlantic ice sheets fennoscandian british isles and icelandic probably surged simultaneously discharging ice and ird into the north atlantic as opposed to theories implying a unique laurentide origin gwiazda et al 1995 driven by an internal mechanism mcayeal 1993 we confirm that the icelandic and the fennoscandian ice sheets also surged as recently proposed by other authors and we here also distinguish a possible detrital contribution from the british isles ice sheet this pan north atlantic phenomenon thus requires a common regional external forcing

variations of okhotsk sea oxygen minimum zone comparison of foraminiferal and sedimentological records for latest mis 12 11c and latest mis 2 1 abstract benthic foraminiferal assemblage compositions foraminiferal and sedimentological proxies were analyzed in core md01 2415 from the northern slope of the okhotsk sea to compare variations of productivity and oxygen minimum zone omz intensity during the latest marine isotope stage mis 12 11c and latest mis 2 1 the benthic assemblages reveal close similarity between the two climatic cycles the absence of benthic assemblages during the latest mis 12 and the presence of the low productivity angulogerina angulosa assemblage during the latest mis 2 suggest the disappearance of the omz this regime was related to almost perennial ice cover with periods of active ice rafting during the latest mis 12 while it was attributed to prolonged seasonal ice cover low surface productivity and enhanced formation of well oxygenated water masses in the okhotsk sea during the latest mis 2 in deglacial times the omz gradually intensified as evidenced by the high productivity uvigerina akitaensis assemblage during the early termination t tv and early ti and the low oxygen tolerant bolivina spissa assemblage during the late tv and late ti the orbitally forced similar rises of the global sea level during tv and ti caused a large offshore supply of organic matter synchronously nutrients from the melting sea ice and shelf erosion promoted high surface mainly carbonate productivity as a result a high and sustained flux of particulate degraded and refractory organic matter enhanced oxygen consumption in bottom waters and sediments leading to the similar gradual omz intensifications the b spissa assemblage also points to expansion of oxygen depleted water mass from the north pacific into the okhotsk sea fostering the omz intensifications the phytodetritus related islandiella norcrossi assemblage indicates weakening of the omz during full interglacial times of mis 11c and mis 1 after stabilization of the global sea level nutrients were mainly delivered by regional upwelling and fluvial discharge favoring increased biogenic opal and carbonate production in the surface water similar to the present in this way moderate to high although less than deglacial and pulsed flux of predominantly particulate organic matter caused the weakening of the oxygen consumption and omz notably during mis 11c the benthic assemblage with the dominance of the taxa with dissolution resistant tests such as miliammina herzensteini karreriella baccata and martinottiella communis reflects carbonate dissolution events in sediments these events might have been resulted from an interruption of the local surface carbonate production and inflow of more carbonate corrosive water masses from the north pacific driven by a drawdown of the global ocean carbonate saturation

40ar 39ar and pb pb study of individual hornblende and feldspar grains from southeastern baffin island glacial sediments implications for the provenance of the heinrich layers lead isotope analysis of individual feldspar grains and 40ar 39ar analysis of individual hornblende grains from three samples taken from baffin island ice proximal till deposits are consistent with a provenance recording a two stage geologic history including formation of continental crust in the late archean and partial isotopic resetting and or reequilibration during major paleoproterozoic regional metamorphism and magmatism significantly although the populations are small and the two stage archean paleoproterozoic history of their sources are similar the pb isotope compositions of samples from baffin island do not match the results from heinrich layer h2 in core hu87 033 009 from northeast of the hudson strait in the labrador sea specifically in h2 samples from hu87 033 009 the 207pb 204pb is high for a given value of 206pb 204pb additionally the range in 206pb 204pb is greater in hu87 033 009 samples and there are two distinct trajectories of 208pb 204pb the steeper of the 208pb 204pb versus 206pb 204pb trends is not seen in the studied baffin island glacial sediment samples samples of feldspar grains from heinrich layers h1 h2 h4 and h5 from core v28 82 in the eastern north atlantic have a 207pb 204pb versus 206pb 204pb relation similar to that of baffin island till samples but there is also a high 208pb 204pb that we have not captured in this suite of glacial sediment samples our results highlight the strength of a combined radiogenic isotope approach to characterizing the geological history of sediment sources and provide a first step in assessing the variability in isotope character in this region of dynamic glacial events during the late quaternary period

final stages in the collapse of the laurentide ice sheet hudson strait canada nwt 14c ams dates seismics stratigraphy and magnetic susceptibility logs hudson strait is a key element in the history of ice sheet ocean interactions during the deglaciation of the laurentide ice sheet data from over 30 giant piston cores supplemented by ca 40 14c ams dates on foraminifers and shells from sites within and adjacent to hudson strait are used to describe the timing and sediment processes associated with the final complex phases of deglaciation in hudson strait each core is placed in a regional seismic architectural context through examination of deep tow huntec profiles most cores sample a thin postglacial unit and extend several meters into glacial marine sediment mainly silty clays 14c dates on cores from the adjacent shelf indicate that deglaciation occurred by 13 ka we have not been unable to confirm that hudson strait was ice free by 11 12 ka although this is a probable scenario whole core magnetic susceptibility wcms data are used as the primary signature of changes in the style of sedimentation and provenance and to correlate between cores wcms indicates that there is a regional differentiation in the ms signal associated with bedrock provenance changes in contribution of detrital carbonate and style of sedimentation one low wcms event is present in cores from both the western and eastern basins it dates from 8 0 ka and provides an important regional stratigraphic marker probably associated with the final deglaciation of hudson bay and hudson strait sediment from a southerly source ungava bay continued to be contributed into the 900 m deep eastern basin of hudson strait 8 and 7 ka and thick postglacial sequences were piled within ungava bay between 6 and 7 ka

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

clay mineral signature of the nw atlantic boundary undercurrent abstract surface sediments were sampled along 2 transects across the iceland and irminger basins and in the labrador sea clay mineral assemblages deduced from x ray analyses of the carbonate free

tracing past ocean circulation to understand how ocean circulation and climate may change in the future we must first understand how the system operated in the past without man9s influence new inorganic isotopic tracers are now helping to elucidate past ocean circulation patterns but controversies remain regarding the relative influence of ocean circulation and weathering on the isotopic signals

isotope stratigraphy sedimentation rates deep circulation and carbonate events in the labrador sea during the last 200 ka high resolution stable isotope and accelerator mass spectrometry 14c measurements in 25 box and piston cores from the labrador sea allow the reconstruction of major paleoceanographical changes during the last 200 ka in this basin and also document the links between the interior of the laurentide ice sheet and the north atlantic ocean during this interval two deep circulation regimes are indicated by contrasting sedimentation rates on the slopes i the modern situation is characterized by a gyre of the north atlantic deep water nadw components into the basin which is responsible for the high energy western boundary undercurrent this regime also prevailed during the former true interglacials and possibly during shorter warm intervals e g bolling allerod ii a more frequent nadw free situation during glacial and interstadial intervals is marked by the presence of a vertically homogeneous water mass in the basin sedimentological records also indicate that carbonate events i e pulses of

tracking the sources of icebergs with lead isotopes the provenance of ice rafted debris in heinrich layer 2 the provenance of ice rafted debris ird deposited in the north atlantic before during and after heinrich event 2 has been determined through measuring the lead isotopic composition of single feldspar grains and multiple grain composites from the larger than 150 m size fraction from cores from the eastern and western north atlantic and from the labrador sea single grain analyses are used to identify the specific continental sources of the ird whereas composite samples are used to assess the relative ird contributions from different sources all single grains from heinrich layer 2 h 2 as well as h 2 composites plot along a correlation line on a 207pb 204pb versus 206pb 204pb diagram characteristic of the churchill province of the canadian shield this is yet another strong piece of evidence that this heinrich event was dominated by a massive iceberg discharge of the laurentide ice sheet lobe located over hudson bay in contrast single grains from the ambient glacial sediment above and below h 2 have multiple sources many of them also lie along the correlation line with h 2 grains but many others have pb signatures consistent with derivation from the grenville province and the appalachian range in north america and possibly from scandinavia and greenland composites from the ambient sediment generally lie well to the right of the h 2 reference line in agreement with the results of the single grain analyses the evidence provided by lead isotopes regarding the dominant role played by the hudson bay lobe of the laurentide ice sheet in the development of the heinrich events lends support to the binge purge model advanced by macayeal 1993a b that invokes trapping of geothermal heat by the base of the icecap and subsequent basal melting as the mechanism that triggered the heinrich events

salinity history of the northern atlantic during the last deglaciation the claim has been made see broecker et al 1988 that production of north atlantic deep water terminated during younger dryas time and that the onset of this termination occurred about 11 000 years ago when the flow of meltwater from a large segment of the southern margin of the laurentide ice sheet was diverted from the mississippi to the st lawrence drainage fairbanks 1989 points out a serious weakness in this argument based on a sea level curve derived from radiocarbon dates on coral obtained from borings made off the barbados coast he suggests that a lull in the melting of the ice caps during younger dryas time may have more than compensated for the impact of the diversion the purpose of this paper is to reassess the situation regarding the origin of the younger dryas in light of this new evidence currently the salinity of surface waters in the northern atlantic is influenced by three fluxes water vapor transport from the atlantic drainage basin to the pacific indian basin tends to raise the salinity of the entire atlantic the excess over evaporation of precipitation and runoff poleward of 40 n tends to reduce the salinity of waters in this region relative to the atlantic average the conveyor circulation of the atlantic trades more salty waters of the atlantic with less salty waters outside the atlantic tending to drive down the atlantic s salinity the conveyor circulation also flushes the northern atlantic pushing its salinity toward the mean for the atlantic during the period of deglaciation meltwater emanating from the laurentide and scandinavian ice sheets was also important this flux tended to lower not only the salinity of the entire atlantic but also the salinity of surface waters in the northern atlantic relative to the atlantic s mean as deepwater formation in the northern atlantic depends critically on the salinity of surface waters the interactions among these fluxes can change the strength of the conveyor

the resilience and sensitivity of northeast atlantic deep water end to overprinting by detrital fluxes over the past 30 000 years abstract the neodymium isotope proxy has become a valuable tool for the reconstruction of past ocean water mass provenance and mixing for its accurate application knowledge about the origin and preservation of nd in sedimentary archives is crucial recently concerns have emerged regarding the applicability of neodymium isotopes as a conservative palaeo water mass tracer given potential nd fluxes from sediments into bottom waters abbott et al 2015a and inferred relabelling of ocean waters by settling detrital material roberts and piotrowski 2015 consequently a decoupling of water mass provenance and proxy variations may arise we investigate the mobility of nd around extreme detrital sedimentation events such as glacial ice rafting pulses and turbidite deposition in the northeast atlantic the constructed records from sediment leachates span extreme nd isotope variations including volcanic end 0 and laurentian end 27 sources we find that nd was released into pore waters from reactive detritus inside some detrital layers during early diagenesis thereby overprinting any archived bottom water nd signature and precluding the reconstruction of past water mass provenance during the affected time intervals however we do not observe any definite indication of diffusive vertical migration of nd into adjacent layers furthermore bottom water nd isotope signatures were not modified to a measurable degree by any potential benthic flux of nd during the deposition of these detrital sediment layers consequently the nd isotope composition of the pelagic glacial northeast atlantic water masses were resilient to such episodic large detrital fluxes apart from extreme local sedimentation events we confirm the presence of detritally overprinted deep waters north of 47 n during the peak glacial from comparison of northeast atlantic depth transects we furthermore suggest that the sensitivity of deep waters to this overprinting effect increased during periods of reduced atlantic meridional overturning circulation and elevated ice rafting overall our study demonstrates that a thorough evaluation of the proportion of nd originating from physical water mass advection versus in situ chemical inputs is crucial for the reliable application of nd isotopes as a water mass tracer

changes in the western boundary undercurrent outflow since the last glacial maximum from smectite illite ratios in deep labrador sea sediments high resolution mineralogical studies were performed on late glacial and deglacial sediments from two deep piston cores from the labrador sea located at the inlet sw greenland rise and outlet labrador rise of the western boundary undercurrent wbuc gyre at the two sites smectites transported from the eastern iceland and irminger basins by the wbuc are observed clay mineral changes are used as proxies for the paleointensity reconstruction of the wbuc on the greenland rise a clay mineral index smectite illite s i ratio is defined a s i ratio of 1 characterized the last glacial maximum it increased after 17 ka and reached a maximum value of 4 during the early holocene the mineralogical changes are gradual and do not show any reversal during the younger dryas this pattern which is confirmed by first order estimations of smectite and illite fluxes suggests gradually increasing sedimentary fluxes and wbuc intensity since the last glacial maximum a peak in the velocity of the wbuc at 9 ka as recorded by clay assemblages is consistent with other regional studies based on pollen foraminifera or grain size measurements a massive dilution of smectites by illite and chlorite s i 3 occurs at 8 5 ka it corresponds to a period of rapid sediment accumulation and reflects an intensified illite rich detrital supply by meltwaters from the southern greenland ice margin on the labrador rise the smectite content varies between 20 and 60 with no obvious trend through time the mineralogical composition is strongly influenced by ice rafted deposition and by the abundance of fast deposit units cf heinrich layers in the north atlantic which contain abundant detrital carbonates spilled over from the north west atlantic mid ocean channel in such layers smectites are present but are diluted by the addition of illites chlorites and kaolinites this provides evidence for a discrete and continuous wbuc supply of fine particles from the irminger and iceland basins as far as the southeastern part of the labrador basin early deglacial smectite rich layers up to 60 are also observed at this site they indicate an increase in the outflow of the wbuc at 13 5 ka bolling allerod as previously reported from grain size or foraminiferal assemblage studies

weathering versus circulation controlled changes in radiogenic isotope tracer composition of the labrador sea and north atlantic deep water geological reconstructions and general circulation models suggest that the onset of both northern hemisphere glaciation 2 7 myr ago and convection of labrador sea water lsw were caused by the closure of the panama gateway 4 5 myr ago time series data that have been obtained from studies of ferromanganese crusts from the northwestern atlantic suggest that radiogenic isotopes of intermediate ocean residence time pb and nd can serve as suitable tracers to reconstruct these events however it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering in this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in lsw but can be explained in terms of increases of erosion levels due to the glaciation of greenland and canada furthermore we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic pb and nonradiogenic nd from continental detritus eroding into the labrador sea this can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic pb should provide such a rich supply of radiogenic pb to the oceans an important general conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources perhaps more importantly the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater

late quaternary iceberg rafted detritus events on the denmark strait southeast greenland continental slope 65 n related to north atlantic heinrich events hu93030 007 lcf 65o01 39 0 n and 30o14 81 0 w is a 4 5 m long giant 11 cm diameter piston core from 1800 m water depth on the continental slope south of the denmark strait sill the core sampled acoustically stratified sediments at a site below the kangerlussuaq trough nine ams 14 c dates on planktic foraminifera indicate that the core spans 14 to 28 14 c ka the sediments consists of two principal lithofacies lithofacies i li comprises tan ochre intervals with abundant ice rafted sandstone siltstone clasts larger than 2 mm and with an average thickness of 47 cm it is also associated with peaks in carbonate content and toc lithofacies ii lii consists of fine grained dark gray hemipelagic sediment there are three distinct intervals of li dated between 14 3 and 15 5 lic 20 1 and 21 9 lib 24 7 and 25 8 lia ka the ages suggest that deposition of li is approximately coeval with heinrich events 1 and 2 in the north atlantic whereas lia appears younger than h 3 lithofacies i is derived from far travelled ice bergs from an unknown source but not from the northeastern laurentide ice sheet stable isotope studies on neogloboquadrina pachydermasinistral show little evidence for significant meltwater events associated with li but surprisingly light oxygen isotopic events are present in the benthic foraminiferal analyses cibicidoides wuellerstorfiand melonis zaandamae the ages of the hu93030 007 iceberg rafted detritus ird events are similar to ird peaks in cores north of denmark strait on the east greenland slope hu87033 009 lcf se baffin slope 62o30 99 0 n and 59o26 82 0 w 1437 m wd is situated in a similar position to hu93030 007 but with respect to the laurentide ice sheet its d 18 o record on planktic foraminifera is virtually identical to that of hu93030 007 the timing of ird events is similar in both these cores between 14 and 30 ka h 3 may be present in hu87033 009 as an ird event but in contrast to h 1 and h 2 it is not evident as a discernible detrital carbonate event 1998 elsevier science b v all rights reserved

smectite composition as a tracer of deep circulation the case of the northern north atlantic abstract the link between smectite composition in sediments from the northern north atlantic and labrador sea and deep circulation is being further investigated through detailed studies of the x ray pattern of smectites and cation saturations this allows clear distinction of dominant terrigenous sources associated to the main components of the modern western boundary undercurrent time variations of smectite characteristics in two piston cores from the inlet and outlet of the western boundary undercurrent gyre in the labrador sea indicate 1 a more southern circulation of north east atlantic deep water during the late glacial 2 a step by step transition to the modern pattern of deep circulation during the late glacial holocene transition with intensification of north east atlantic deep water and davis strait overflow 3 an expansion of davis strait overflow and labrador sea water circulation in relation to ice surges and deposition of detrital layers 4 an intensified circulation of north east atlantic deep water during the younger dryas and 5 a very recent increased influence of denmark strait overflow water beginning between 4 4 and

circulation in the glacial north atlantic inferred from grain size measurements records of nutrient proxies in marine sediments indicate that the nutrient distribution and hence circulation of the glacial north atlantic ocean was markedly different from that of today1 2 but these tracers are influenced by several biogeochemical factors unrelated to ocean circulation3 6 and thus do not provide a direct measure of the vigour of circulation distributions of grain size in marine sediments can provide such information owing to the sorting effects of currents7 8 if the characteristics of the input sediment flux are known here we present a record inferred from grain size measurements of variations in the relative strength of deep and intermediate currents in the eastern north atlantic over the past 25 000 years we find that glacial intermediate water flowed rapidly at depths of between 1 100 and 2 000 m in contrast deep water circulation was sluggish during the last glaciation but increased in strength shortly after the glacial maximum this would have resulted in increased heat flux to high latitudes and may have triggered sudden deglaciation deep current strengths declined again at the start of termination stage ia and during the younger dryas perhaps as a result of iceberg discharges the so called heinrich events9 10 a remarkably similar record of grain size variations has been found in the western north atlantic11 indicating that these changes in circulation are ocean wide

sr_nd_pb isotope systematics in amazon and congo river systems constraints about erosion processes abstract 87 sr 86 sr 143 nd 144 nd 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb isotopic ratios and rb sr sm nd u pb and th concentrations have been measured in the suspended loads of the congo and amazon rivers and their tributaries in the dissolved load 87 sr 86 sr rb sr nd sm u pb and th concentrations are also reported these results show that nd sm th and pb are almost insoluble and that their mass balance is controlled by particulates whereas rb sr and u are fractionated between soluble and particulate phases the 87 sr 86 sr ratios can only be interpreted after computing the amount of carbonate recycling and the partitioning for silicates between soluble and insoluble this paper presents a method based on the lead isotopic system that can be used to test the steady state of erosion which is tacitly assumed in many river and erosion studies the results presented show that the steady state is validated in each river of the congo basin and in the lowland rivers of the amazon basin but in not verified in the rivers from the andes rio solimoes and rio madeira 87 sr 86 sr 206 pb 204 pb and 208 pb 204 pb ratios are positively correlated and negatively correlated with 143 nd 144 nd ratios a number of arguments indicate that these correlations are mixing lines the end members being orogenic and shield components respectively for the amazon basin analysis of the river loads shows that the orogenic zone is favoured some 5 times more than the shield on an equal area basis this leads to a reinterpretation of sm nd data for shales and casts some doubts upon the proposition of secular variations in the sm nd ratio of the continental crust

rapid climatic variations during marine isotopic stage 3 magnetic analysis of sediments from nordic seas and north atlantic abstract the bulk magnetic parameters of seven deep sea cores distributed from the nordic seas 67 n to the north atlantic as far south as the bermuda rise 33 n exhibit short term variations which correlate with rapid climatic changes during marine isotopic stage 3 mis3 the magnetic mineralogy is uniformly dominated by well sorted low ti content magnetites indicating that these variations are due to variations in the relative amount of magnetic minerals because the magnetic minerals predominantly originate from one common source area the nordic basaltic province these changes arise from changes in the efficiency of the transport of the magnetic particles by deep currents from the source to the site of deposition these results therefore show that the fast climatic changes are related to coeval fast changes in the strength of the deep sea circulation the latter was active reduced during the interstadials stadials and heinrich events transporting the magnetic particles from the norwegian sea into the north atlantic ocean along a path similar to the present path of the nadw it is tentatively suggested that the faeroe shetland channel and the denmark strait were the only two active paths for the overflow water during mis3 the presence of magnetic oscillations in the bermuda rise core in phase with those from the north atlantic indicates that the activity of the southern newfoundland basin gyre was linked to that of the nadw during mis3

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

nd isotopes in indian ocean sediments used as a tracer of supply to the ocean and circulation paths abstract neodynium isotopic compositions of eighteen sediments from the indian ocean were measured these quaternary marine sediments were collected in different physiographic provinces with samples being taken from the abyssal plain ridge submarine fan fracture zone and aseismic ridge environments the main results is that there are large areas in the indian ocean which are isotopically homogeneous although the results also enforce the idea that in marine sediments the continent derived material exerts an important control on the nd budget the northeastern part of the indian ocean is influenced by erosion of the indonesian arc the values between e 0 7 and e 0 8 observed in samples from the central indian ocean are nearly constant both in the eastern and western parts of the indian ocean this was interpreted as mixing between two major inputs firstly material coming from the erosion of the indian shield e 0 14 to 17 and secondly from the indonesian arc e 0 0 subsequently all the material is then homogenized by the oceanic circulation the western indian ocean values are more variable perhaps because of the proximity of old continental terrains i e southern africa and the influence of atlantic waters the highest value e 0 2 93 observed in the southern part of the indian ocean may be explained by the influence of the erosion of the crozet basaltic plateau indeed the two samples which exhibit positive nd isotopic values correspond geographically to the area of erosion of the crozet and possibly the kerguelen basaltic plateaus the location of the sample with the lowest isotopic value e 0 18 88 close to antarctica implies that this sediment might have included an old continental component of precambrian or even archaen age this result traces in the case of particle transport the circulation of water masses from antarctica to northern parts of the indian ocean which are known for their deep waters with the exception of the samples located near the crozet plateau where probable destruction of the plateau is occurring we do not observe any nd isotopic variations related to the specific physiographic location of any one sample consequently the nd isotopic variations are principally due to the pathways of water circulation and possibly winds over the indian ocean

deep circulation changes in the labrador sea since the last glacial maximum new constraints from sm nd data on sediments sm nd concentrations and nd isotopes were investigated in the fine fraction of two labrador sea cores to reconstruct the deep circulation patterns through changes in sedimentary supply since the last glacial stage three sources are involved the north american shield palaeozoic rocks from northeastern greenland and mid atlantic volcanism the variable input of these sources provides constraints on the relative sedimentary supply in conjunction with inception of deep currents during the last glacial stage a persistent but sluggish current occurred inside the labrador basin an increasing discharge of volcanic material driven by the north east atlantic deep water is documented since 14 3 kyr signaling the setup of a modern like deep circulation pattern throughout the labrador irminger and iceland basins during the last deglacial stage the isotopic record was punctually influenced by erosion processes related mainly to ice sheet instabilities especially 11 4 10 2 and 9 2 kyr ago

sources of labrador sea sediments since the last glacial maximum inferred from nd pb isotopes pb isotopes have been measured in the clay size fraction of late glacial and holocene deep sea sediments recovered from two labrador sea piston cores that have been previously analyzed for nd isotopes the newly acquired pb isotopic data allow us to better constrain the different source areas that supplied clay size material during the last deglaciation until 8 6 kyr calendar ages nd pb data can be modeled mainly as a mixture between a precambrian crust and lower paleozoic material originating from the denudation of the pan african orogen the old material originates mainly from the archean lower proterozoic or both terranes of greenland and also probably corresponding terranes of labrador although minor input of other precambrian material is recorded in some detrital carbonate rich deglacial samples from orphan knoll the phanerozoic crustal end member consists of sediment material mainly originating from northwestern europe this source area is found to be the only significant source of young crustal material in early holocene sediments from the greenland rise no significant input from the mid atlantic volcanism is apparent this study puts further constraints on the deep circulation pattern during the last deglaciation it is concluded that at that time european phanerozoic material was carried from the norwegian sea through the wyville thompson ridge into the iceland basin by the north east atlantic deep water no evidence for an overflow is found either south of the iceland iceland scotland ridge or through the denmark strait

a 200 ka geomagnetic chronostratigraphy for the labrador sea indirect correlation of the sediment record to specmap abstract a chronostratigraphic framework for the last 200 ka based on relative variations of geomagnetic paleointensity has been derived for the labrador sea where 18o stratigraphies are compromised by lack of benthic foraminifera and environmental perturbations of the planktic record relative paleointensity records derived from three labrador sea piston cores are correlated to a recently developed 200 ka paleointensity composite sint 200 y guyodo j p valet earth planet sci lett 143 1996 23 36 the chronology for sint 200 was derived by correlating oxygen isotope stratigraphies from individual paleointensity records to the specmap chronostratigraphy the correlation of sint 200 to labrador sea paleointensity records allows the specmap chronology to be imported into the labrador sea although comparisons of the chronostratigraphy based on paleointensity correlations with those derived from ams 14c dates and 18o data indicate significant discrepancies support for the new chronostratigraphic framework is provided by 1 consistent correlation of labrador sea detrital layers to north atlantic heinrich layers 2 general consistency between planktic 18o values and the specmap reference curve and 3 feasible prediction for the age of ash layer 2 as the new chronostratigraphy is tied to specmap labrador sea sediment cores can now be directly compared with any specmap dated record

effects of sedimentary sorting on neodymium isotopes in deep sea turbidites the study of neodymium model ages of sedimentary rocks has added considerably to our understanding of crustal growth and evolution so far however the possible effects of various sedimentary processes on nd isotope distributions have not been fully evaluated here we present nd isotope data for a series of sand mud pairs in modern deep sea turbidites from different tectonic settings which show that sands and muds commonly have significantly different nd isotope compositions that in only some cases are associated with differences in sm nd ratio differences are as great as 7 ndunits corresponding to nd model ages that differ by up to 0 44 gyr and may be explained by separation or unmixing of components from different sources during sedimentary transport and sorting for one sample heavy mineral fraction ation may be indicated these results have implications for studies of nd model ages in sedimentary rocks and for the interpretation of provenance from sedimentary petrography and geochemistry

the magnetic signature of rapidly deposited detrital layers from the deep labrador sea relationship to north atlantic heinrich layers rock magnetic parameters from deep labrador sea piston core p 094 are useful for recognizing rapidly deposited detrital layers some of which correlate with north atlantic heinrich layers variations in magnetic properties and lithology distinguish two classes of rapidly deposited detrital layers during the last ice age 1 seven detrital carbonate dc layers have high carbonate content and mean magnetite grain diameters about 4 times greater than the background sediments 2 six low detrital carbonate ldc layers have very similar magnetic properties to dc layers but low carbonate content similar to the background sediments the magnetite associated with dc and ldc layers is well sorted and relatively uniform within and between layers ice rafted detritus ird recognized by increased percentage of the 125 micron grain size fraction and by high magnetic susceptibility is associated with most dc and ldc layers but is not the dominant detrital constituent the correspondence of dc and ldc layers with increased grain size of well sorted magnetite but not with coarse fraction content precludes ice rafting as the primary depositional mechanism dc and ldc layers may have been deposited from suspended sediment derived from turbidite activity in the nearby northwest atlantic mid ocean channel namoc this interpretation is supported by a piston core p 013 from a greenland rise site largely but not completely outside the influence of the namoc the association of dc layers with ird and the apparent age correlation of dc layers to north atlantic heinrich layers suggest that the ice advances which produced the ird in p 094 and in correlative north atlantic heinrich layers also triggered turbiditic flows down the namoc several dc and ldc layers however do not correlate with recognized heinrich layers but appear to be coeval with recently discovered high frequency ird rich layers in the north atlantic

smnd signature of modern and late quaternary sediments from the northwest north atlantic implications for deep current changes since the last glacial maximum abstract nd isotopes and sm nd concentrations have been investigated in the clay size fraction of surface and late quaternary sediments of the northern north atlantic four sources of nd have been identified in the surface sediments 1 a radiogenic nd typical of the iceland volcanism 2 a pan african nd issued from eastern greenland 3 a variscan nd derived from the arctic ocean 4 an old unradiogenic nd related to the north american shield in the iceland and irminger basins the major sources of nd are the variscan nd derived from the arctic ocean and the iceland nd except in the western irminger basin where the pan african nd originating from eastern greenland is dominant and labels sedimentary supplies linked to the denmark strait overflow water nd derived from icelandic volcanism is never clearly dominant even on the reykjanes ridge and the nd derived from the american shield becomes dominant only in the labrador sea sediments lying in the pathway of the western boundary undercurrent have nd isotopic compositions which are shifted towards more radiogenic values indicating an input of material from the eastern basins carried by this deep current late quaternary sediments were cored from the southern greenland rise core 90 013 013 and eastern labrador rise orphan knoll core 91 045 094 in core 90 013 013 nd values during the last glacial maximum are as low as 20 but subsequently rose to 9 in the modern sediments indicating that the sedimentary material during the last glacial maximum was dominated by unradiogenic nd from the north american shield this may be related to a lesser outflow of the western boundary undercurrent in core 91 045 094 located along the labrador current trajectory a dominant supply of north american shield nd is recorded perhaps originating at least partially from the hudson strait the sm nd data are compatible with smectite vs illite studies at the same sites which suggest a continuous but more modest western boundary undercurrent supply during last glacial maximum

paleoceanography of the deep western boundary undercurrent on the north american continental margin for the past 25 000 yr temporal fluctuations of silt mean particle size and foraminiferal indices were used to examine fluctuations in velocity and position of the axis of the deep western boundary undercurrent dwbuc on the north american continental margin from 25 to 17 ka the dwbuc axis was 500 m shallower and farther inshore than its present position on the lower rise current speed was high from 25 to 20 ka and then waned until 17 ka between 17 and 7 ka the major axis of the dwbuc shallowed to 4000 m current speed increased from 17 to 11 ka but slowed from h to 7 ka since 7 ka the dwbuc axis has deepened to its modern position at 4900 m the zone of maximum carbonate dissolution remained below the dwbuc the most intense dissolution was from 22 to 9 ka and 5 to 2 ka highest abundance of uvigerina and highest percent benthic foraminifera were immediately above the dwbuc from 22 to 7 ka after which values of both parameters fell to near zero

south asian monsoon history over the past 60 kyr recorded by radiogenic isotopes and clay mineral assemblages in the andaman sea the late quaternary variability of the south asian or indian monsoon has been linked with glacial interglacial and millennial scale climatic changes but past rainfall intensity in the river catchments draining into the andaman sea remains poorly constrained here we use radiogenic sr nd and pb isotope compositions of the detrital clay size fraction and clay mineral assemblages obtained from sediment core nghp site 17 in the andaman sea to reconstruct the variability of the south asian monsoon during the past 60 kyr over this time interval end values changed little generally oscillating between 27 3 and 25 3 and the pb isotope signatures are essentially invariable which is in contrast to a record located further northeast in the andaman sea this indicates that the source of the detrital clays did not change significantly during the last glacial and deglaciation suggesting the monsoon was spatially stable the most likely source region is the irrawaddy river catchment including the indo burman ranges with a possible minor contribution from the andaman islands high smectite illite1chlorite ratios up to 14 as well as low 87sr 86sr ratios 0 711 for the holocene period indicate enhanced chemical weathering and a stronger south asian monsoon compared to marine oxygen isotope stages 2 and 3 short smectite poor intervals exhibit markedly radiogenic sr isotope compositions and document weakening of the south asian monsoon which may have been linked to short term northern atlantic climate variability on millennial time scales

investigation of a possible submarine landslide at the var delta front nice continental slope southeast france the var prodelta progrades across a straight narrow shelf less than 2 km wide with a very steep continental slope reaching locally more than 30 historically the var delta front is sadly famou

productivite et flux de carbone dans la mer du labrador au cours des derniers 40 000 ans geochemical and micropaleontological analyses of labrador sea sediments allowed us to intercalibrate indicators of paleoproductivity and paleofluxes of carbon and to reconstitute biogeochemical fluxes notably in relation to deep sea circulation changes western boundary undercurrent and north atlantic deep water linear or logarithmic relations are observed between the subrecent fluxes of paleoproductivity indicators coccoliths diatoms dinocysts 230th the 13c content of left coiling or sinistral neogloboquadrina pachyderma the modern surface water carbon production and the rates of organic and inorganic carbon corg cinorg burial all indicators increase by a factor of 102 to 103 in response to the evolution of paleosea surface conditions since the last glacial maximum correlations are observed between authigenic cinorg 230th coccoliths and 13c content left coiling or sinistral neogloboquadrina pachyderma and also between authigenic corg dinocysts uranium and sulfur relative cinorg

provenance of heinrich layers in core v28 82 northeastern atlantic 40ar 39ar ages of ice rafted hornblende pb isotopes in feldspar grains and nd sr pb isotopes in the fine sediment fraction abstract several correlatable layers of sediment rich in ice rafted grains have been documented in the north atlantic the most notable within the last glacial cycle are the heinrich layers layers extremely rich in ice rafted detritus and generally barren of foraminifera within the north atlantic ice rafted detritus ird belt the view of these layers is that they represent events where great armadas of icebergs were launched into the north atlantic the importance of the heinrich layers lies in their connection with abrupt climate change in the north atlantic and perhaps globally there is a growing number of published provenance studies of the heinrich layers in the north atlantic based on a variety of methods however there is little overlap of methods applied to the same samples in this contribution we present a multi component provenance study of heinrich layers h1 h2 h4 and h5 from core v28 82 in the eastern north atlantic our results indicate that virtually the entire inventory of terrigenous clastic detritus in heinrich layers h2 h4 and h5 came from ancient continental sources surrounding the labrador sea although heinrich layer h1 is similar in many respects it appears to have some significant differences relative to the other three

grain size and srnd isotopes as tracer of paleo bottom current strength northeast atlantic ocean abstract a crucial question in modern paleoceanographic research is how vigourous was the bottom current in the glacial north atlantic ocean distribution of sediment grain size can provide such information if the characteristics of the source functions are known the interest of this paper is to use the sr nd isotopic composition of lithic particles as a tracer of origin in order to determine possible changes in the source function over the last 185 kyr grain size distribution data are derived from a sediment core in the icelandic basin gardar drift the distribution pattern reveals a well sorted silt size modal size 20 m in the glacial deposits sr and nd isotopic composition measured on separate size fractions indicates a more constant local origin iceland for the silt fraction than for the fine fraction these data suggest that the increase in the silt fraction during glacial periods is related primarily to an increase in the bottom current strength rather than to changes in sediment supply i e ice rafting turbidity current the increase in vigour of the bottom current recorded during the isotopic stages 6 4 and the last post glacial periods could have been initiated by a southward shift of the zone of deep water formation in the northern part of the icelandic basin

changes in the strength of the iceland scotland overflow water in the last 200 000 years evidence from magnetic anisotropy analysis of core su90 33 abstract results from a high resolution study of magnetic anisotropy combined with mineral magnetic analysis clay analysis and oxygen isotopes stratigraphy are reported for core su90 33 located 60 34 4n 22 05 1w at 2400 m water depth along the iceland scotland overflow water isow a branch of the north atlantic deep water nadw this core covers the last 6 climatic stages the main magnetic mineral is low ti content magnetite with very slight changes in the grain size the susceptibility record corrected for the carbonate content indicates down core variations in the amount of magnetite which largely co vary with the changes in the proportion of smectite in the clay fraction both parameters are climatically controlled with lower values during glacial than during interglacial periods the measurements of the anisotropy of magnetic susceptibility document down core changes in the degree of anisotropy with significantly higher values during interglacial periods than during glacial times these changes appear to be related to differences in the degree of alignment of the magnetic particles and not to changes in grain shape providing evidence that they arise from changes in strength of the bottom water circulation therefore the strength of the contour current associated to the transport of the isow appears to have been significantly larger during climatic stages 5 3 and 1 than during stages 6 4 and 2

interglacial responses of the southern greenland ice sheet over the last 430 000 years determined using particle size specific magnetic and isotopic tracers abstract the past behavior of the greenland ice sheet can provide important insight into climatic thresholds that may initiate and drive major ice sheet retreat particle size specific magnetic and sr nd pb isotope records from eirik ridge sediments south of greenland track southern greenland ice sheet sgis erosional signatures over the past 430 ka by discriminating changes in sediment source and transport over the eirik ridge ground truthed magnetic and isotopic compositions of subglacial silt from south greenland s precambrian bedrock terranes constrain independent magnetic and isotopic estimates of eirik ridge silt provenance which in turn indicate that the southern greenland ice sheet sgis retreated within its present margin during three of the four previous interglaciations over the past 430 ka retreat of the sgis was extensive during the marine isotope stage mis 5e 9 and 11 interglaciations continuing unabated despite declining insolation during mis 5e and 9 with near complete deglaciation in mis 11 retreat of the sgis during mis 7 was minimal notwithstanding strong insolation forcing while holocene retreat slowed shortly after peak insolation the reconstruction of sgis retreat during the last five deglacial and interglacial periods suggests that a threshold for extensive sgis retreat exists between the insolation and co 2 states of the holocene and the mis 5e and 9 interglaciations with co 2 exerting a stronger control on sgis retreat than insolation our results also suggest that the extent and stability of the sgis in the holocene is anomalous in the context of late quaternary interglaciations

strontium neodymium and lead isotope variations of authigenic and silicate sediment components from the late cenozoic arctic ocean implications for sediment provenance and the source of trace metals in seawater proven ance changes of silicate sediment deposited during the late cenozoic 5 0 ma on the alpha ridge central arctic ocean are determined from variations in strontium lead and neodymium isotope compositions whereas strontium and lead isotope compositions are relatively invariant from 5 to 1 7 ma end values start to increase at 3 ma subsequently s7sr 86sr and 2 6pb z 4pb ratios progressively increase and end values progressively decrease from 1 7 ma to the present day from these isotope variations three different endmember compositions for sediment source regions are defined the two endmember compositions defined by sediment that was deposited by sea ice from 5 to 1 7 ma are consistent with a significant component being derived from the east siberian shelf the progressive change in isotope compositions from 1 7 to 0 ma correlates with an increase in coarse detritus deposited by icebergs over this time period the isotope data are consistent with a progressive increase in the proportion of sediment that was deposited in the central arctic ocean since 1 7 ma having been derived from the northern canada or queen elizabeth island region changes in the strontium neodymium and lead isotope compositions of arctic seawater are determined by analyzing the oxide fractions of fe mn micronodules strontium isotope compositions of the microno dule oxide fractions are similar to published late cenozoic seawater data and indicate that 97 of the sr is seawater derived but minute contributions of sr from silicate detritus prohibit using the hydrogenous fractions for chronostratigraphic purposes neodymium and lead isotope compositions of the micronodule oxide fractions which reflect those of arctic seawater follow the isotope variations of the silicate components throughout the late cenozoic these relations indicate that river water can not be the primary source of rare earth elements and pb to the dissolved reservoir in arctic seawater and that ice rafted detritus by dissolution or exchange processes is an important source of trace elements to seawater in ice covered oceans copyright 1997 elsevier science ltd

microparticle concentration variations linked with climatic change evidence from polar ice cores the microparticle concentrations in three deep ice cores reveal a substantial increase in the concentration of insoluble particles in the global atmosphere during the latter part of the last major glaciation the ratio of the average particle concentration in the late glacial strata to that in the holocene strata is 6 1 for the core from dome c antarctica 3 1 for the core from byrd station antarctica and 12 1 for the core from camp century greenland whether this temporal correlation between increased atmospheric particle load and the lower surface temperatures is directly causal is unknown however the variations in these two parameters must be satisfactorily resolved in any successful hypothesis that addresses the causes of climatic change

provenance of icebergs during heinrich event 3 and the contrast to their sources during other heinrich episodes the sources of ice rafted debris ird from heinrich layer 3 h 3 in the north atlantic were studied by means of pb isotopic analyses of single and multiple grain samples of ice rafted feldspars from core v28 82 49 27 n 22 16 w pb ratios of h 3 grains point to multiple continental sources and are similar to the isotopic composition of the background sediment this similarity as well as previous observations about the age and the sr and nd isotopic composition of the fine fraction of h 3 in cores of the same area are all consistent with the hypothesis that in the eastern north atlantic along the latitude band of maximum ird deposition 40 50 n h 3 is a product of foraminifera dissolution and or low productivity in contrast analyses of composite samples of h 1 h 2 h 4 and h 5 plot in 206pb 204pb versus 206pb 204pb coordinates along the churchill province line supporting the premise that the hudson strait was the major pathway of iceberg drainage during these four events since an advance of the laurentide ice sheet during h 3 is demonstrated by increases in detrital carbonate at the level of h 3 in cores from the western north atlantic the results obtained here suggest that if indeed that advance took place the iceberg discharge melted mostly in the western basin

recirculating components to the deep boundary current of the northern north atlantic abstract the meridional overturning system of the north atlantic is conventionally thought of as transporting warm water 4 c to high latitudes and covd water to low latitudes the northern cold water sources for this system are dense overflow from the nordic seas and less dense water from the labrador sea lsw the overflow components are carried westward by a deep northern boundary current dnbc that is shown here actually to begin southeast of the overflow regions the dnbc lies along the southern side of a system of islands and submarine ridges that divides the nordic seas from the subpolar basins the overflows act to increase the transport of the dnbc and this transport is further increased through the entrainment of warm waters once inside the labrador sea the dnbc is joined by lsw before continuing south to low latitudes as a cold deep western boundary current dwbc however published transport estimates are larger than can be explained by warm water entrainment alone thus indicating entrainment also of recirculating cold components the subject of this paper their sources are shown to be the lsw and cold abyssal waters originating from the southern hemisphere the lsw is entrained downward into the denser part of the dnbc and laterally into the upper part of the dwbc whereas the cold abyssal waters are supplied by an eastern intensified northward flow in the eastern atlantic that serves as the initial source of the dnbc and by a similar northward flow in the western atlantic the meridional overturning system described includes recirculations and poleward transport of cold water in addition tothe components described by the conventional system the cold abyssal waters in both the eastern and western basins have relatively low levels of oxygen and high concentrations of silicate reflecting their southern origin but their influence in the northwestern atlantic is somewhat obscurred by strong recirculating cyclonic gyres in the newfoundland and labrador basins this influence is detected by a deep silicate maximum extending poleward from the mid latitude western atlantic through the two gyres with eastern concentration and recirculating back diluted toward the south in the dwbc within the respective basins such a maximum also extends northward through the irminger basin into the dnbc but there is an ambiguity as to whether this is a direct extension of the signal from the south in the western basin or if it comes from the mid latitude eastern basin by way of westward flow through the charlie gibbs fracture zone it is perhaps consequence of both estimates of the volume transports of the various recirculating cold components indicate that they supply about as much water to the deep boundary currents as do the combined cold water sources in the north

use of stable lead isotopes to characterize the sources of anthropogenic lead in north atlantic surface waters stable lead isotopes are used to illustrate the impact of surface water circulation on dissolved lead distribution in north atlantic surface waters during oligotrophic conditions using stable lead isotopic signatures from 1 the sargasso sea and 2 direct tropospheric deposition to the north atlantic we estimate that 10 40 of the lead accumulated in surface waters of the european basin is transported from the western north atlantic by the north atlantic current south of 50 n lead appears to be primarily distributed by the subtropical north atlantic gyre that extends well beyond the western basins to 30 w in the north african basin at 30 40 n there are different lead isotopic signatures between the subtropical gyre to the guiana and western guinea basins which suggests that the inter tropical convergence zone acts as an efficient barrier limiting chemical exchanges between the gyre and the equatorial currents

pb isotopes and geochemical monitoring of arctic sedimentary supplies and water mass export through fram strait since the last glacial maximum 1 elemental and pb isotope measurements were performed on leachates and residues from surface sediments and two 50 cm cores mc04 and mc16 collected along a ne sw transect through fram strait geochemical and isotopic properties of residues from surface sediments define three distinct spatial domains within the strait 1 the easternmost edge of the strait 2 the eastern part of the strait off the svalbard margins and 3 the western part of the strait influenced by supplies from svalbard the nordic seas with possible contributions from northwestern siberian margins and sea ice and water outflow from the arctic respectively core mc16 in the third domain beneath the outflowing arctic waters spans the last glacial maximum present interval sediments from this core were leached to obtain detrital residues and exchangeable leachates fractions detrital supplies to core mc16 are believed to originate mainly from melting of the overlying sea ice and thus can be used to document changes in arctic sedimentary sources detrital 206pb 204pb and 208pb 206pb ratios illustrate two mixing trends trends a and b corresponding to the pre and post younger dryas yd intervals respectively these trends represent binary mixtures with a common end member canadian margins and either a siberian trend a or greenland trend b margin end member the yd is marked by an isotopic excursion toward the canadian end member suggesting a very active beaufort gyre possibly triggered by massive drainage of the laurentide ice sheet pb isotope compositions of leachates thought to represent the signature of the overlying water masses define a unique linear trend coincident with trend a this suggests that water masses acquired their signature through exchange with particulate fluxes along the canadian and siberian continental margins

authigenic pb isotopes from the laurentian fan changes in chemical weathering and patterns of north american freshwater runoff during the last deglaciation climate dependent chemical weathering trends have a strong impact on the dissolved pb isotopic composition of continental runoff during glacial terminations so that this tracer can be used to reconstruct the impact of the north american deglaciation on aspects of freshwater runoff and ocean chemistry here we present authigenic fe mn oxyhydroxide derived pb isotope records from lower laurentian fan sediments that trace the local deglacial continental runoff signal through the gulf of st lawrence we use these records to investigate changes in the pb isotopic composition of the north american runoff and their relationship to deglacial processes the new pb isotope records are very similar to those from the subtropical deep nw atlantic locations though the new data are at higher resolution and exhibit much greater amplitudes of change due to their laurentide ice sheet proximal setting unradiogenic compositions 206pb 204pb as low as 18 8 at 18 5 ka during the latest glacial change towards highly radiogenic compositions during the early holocene 206pb 204pb 20 1 at 11 2 ka late holocene 206pb 204pb values return to intermediate values around 206pb 204pb 19 6 these new data support previous inferences that the pb isotopic composition of the deep atlantic is controlled during deglaciation by changes in the pattern and intensity of continental chemical weathering in more detail the most significant change in the entire pb isotope record starts at 12 5 ka and lasts until about 11 ka this observation of strong pb isotopic changes across a climatically cold period in the north atlantic region suggests that at this location pb isotopic compositions recorded in deep marine authigenic fe mn oxyhydroxides not only trace deglacial changes in chemical weathering intensity but also highly depend on major freshwater runoff routes in interior north america our data suggest a gradual opening of the eastward freshwater runoff route in the course of the younger dryas

sm nd isotope systematics in deep sea sediments clay size versus coarser fractions sm nd studies were carried out on last glacial and holocene sediments from the labrador sea three grain size fractions were analyzed for each sediment the clay size fraction 2 m the fine cohesive silts 2 10 m and the coarse silts 10 63 m in most cases sm nd s ignatures are different from one grain size to anot her in addition different sm nd signatures are derived fr om the clay size fractions compared to the cohesive silt fraction indicating that the cohesive fraction is heterogeneous and that distinct origins in terms of source areas are implied silt fractions record the influence of supplies from the north american precambrian shield whereas clay size fractions 2 m are more sensitive to s edimentary input from the eastern basins in relati on with deep currents coupling sm nd studies on silt and c lay fractions may be a powerful tool to unravel the evolution of deep circulations versus detrital supplies

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

weathering fluxes and sediment provenance on the sw scottish shelf during the last deglaciation the reconstruction of past ice sheet dynamics can shed a light on long term ice stream activity and in turn provide constraints on the response of modern ice sheets to climate change the hebrides ice stream his flowed across part of the western scottish shelf to the shelf break during the last glacial cycle and drained a large portion of the northern sector of the british irish ice sheet to investigate the deglacial dynamics of the his following the last glacial maximum lead pb isotope records were extracted from the femn oxyhydroxide and detrital fractions of recovered laminated glacimarine mud sequences to monitor the changing activity of his during its retreat these provide timing and some source information of glacially weathered inputs to the marine environment the femn oxyhydroxide fraction in the samples is dominated by allochthonous particles pre formed and shows a marked decrease from radiogenic 20 05 206pb 204pb at 21 cal ka bp to less radiogenic pb isotope compositions 19 48 towards the windermere interstadial 15 4 13 ka this decrease represents a reduction in the flux of subglacially derived radiogenic pb to the continental shelf is interpreted as being associated with the break up of the ice stream in western scotland around that time the pb sr and nd isotopic signatures of the detrital fraction indicate a preponderance of moine sourced fine sediments originated from the nw highlands in the core locations from 21 to 15 cal ka bp dimlington stadial windermere interstadial most likely dictated by the orientation of ice flow tidal and oceanic current directions and sediment delivery in contrast ice rafted debris in a 21 cal ka bp old basal diamicton contains volcanic derived material suggesting different provenance for different grain sizes the femn oxyhydroxide 208pb 204pb ratio shows an unusual inversion relative to the other pb isotope ratios and is attributed to the introduction of secondary weathering phases from a source with contrasting 208pb 204pb but similar 206pb 204pb and 207pb 204pb in the detrital fraction the inversions are constrained to periodic spikes which may indicate an increased contribution from a high th u source potentially the neighbouring archaean amphibolitic lewisian basement in the outer hebrides this study demonstrates how geochemical investigation on continental shelves can be used to constrain the timing activity and flow sources of palaeo ice streams