crustal velocity structure of the omineca belt southeastern canadian cordillera travel time inversion and amplitude modeling of a 350 km lithoprobe seismic refraction wide angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the omineca belt of the canadian cordillera the upper crust to 12 18 km depth has velocities from 5 6 to 6 2 km s 1 and two shear zones the monashee decollement and gwillim creek shear zone are imaged by the wide angle reflections and velocity trends minor velocity differences on either side of the monashee decollement may be related to separate rock origins prominent reflections define the boundaries of a low velocity midcrustal layer from 10 15 km to 20 25 km depth with velocities less than 6 1 km s 1 the low velocities of the midcrust associated with high electrical conductivities and high heat flow may be considered as support for the hypothesis of fluids in the cordilleran crust though other possibilities such as the effect of high temperatures on rock velocities are possible in the lower crust velocities range from 6 4 6 5 km s 1 at the top of the lower crust to 6 6 6 8 km s 1 at its base the moho is very clearly defined by the refraction wide angle reflection data and has a gentle southerly dip crustal thicknesses are 35 37 km a thin crust mantle transition zone of 1 2 km thickness in which velocities vary between 7 6 and 7 7 km s 1 is consistent with coincident reflection data upper mantle velocities range from 7 9 to 8 1 km s 1 with indications from the data of upper mantle layering in comparison with neighboring regions the omineca belt has an anomalously thin crust low crustal velocities and a low velocity upper mantle similar only to the basin and range province the velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method the characteristics of a thin crust and lithosphere along with low velocities from midcrust to mantle suggests that both the basin and range and the southern canadian cordillera are currently being heated from a source within the mantle

episodic reductions in bottom water currents since the last ice age although north atlantic deep water formation was greatly reduced during the last glacial maximum bottom water currents were as vigorous as at present however they were weakened during periods of north atlantic surface freshening a strong correlation can be seen between bottom water current strength and greenland air temperature records thus confirming a close connection between ocean circulation and abrupt climate change past changes in the freshwater balance of the surface north atlantic ocean are thought to have influenced the rate of deep water formation and consequently climate1 2 although water mass proxies are generally consistent with an impact of freshwater input on meridional overturning circulation3 there has been little dynamic evidence to support this linkage here we present a 25 000 year record of variations in sediment grain size from south of iceland which indicates vigorous bottom water currents during both the last glacial maximum and the holocene period together with reconstructions of north atlantic water mass distribution vigorous bottom currents suggest a shorter residence time of northern source waters during the last glacial maximum relative to the holocene period the most significant reductions in flow strength occur during periods that have been associated with freshening of the surface north atlantic the short term deglacial oscillations in bottom current strength are closely coupled to changes in greenland air temperature with a minimum during the younger dryas cold reversal and a maximum at the time of rapid warming at the onset of the holocene our results support a strong connection between ocean circulation and rapid climate change

the magnetic fraction a tracer of deep water circulation in the north atlantic abstract we report on a mineral magnetic study of a series of holocene marine sedimentary sequences distributed from northern to southern bjorn and gardar drifts 56 n to 61 n and in the charlie gibbs fracture zone about 53 n all the cores are located at water depths bathed by the iceland scotland overflow water isow a precursor of the north atlantic ocean deep water formed in the nordic seas and flowing into northeast north atlantic after passing over the volcanic sills between iceland and scotland the magnetic fraction is composed of low ti content magnetites with a grain size distribution in the pseudo single domain range these grains originate from a unique source in the north through the erosion by the bottom waters of the icelandic basaltic province the magnetic grain size remains fairly constant in each core but on average progressively fines towards the south and downstream likewise the magnetic concentration and mean magnetic flux decrease towards the south the spatial evolution of the magnetic concentration and of the magnetic grain size suggests a transport of these magnetites by bottom currents associated with the overflow water downstream from the source with progressive deposition of this magnetic fraction along the path of the isow this study provides evidence that characterization of the magnetic parameters of sediments deposited in such an oceanographic context of a unique source upstream and a well defined water mass may be used to detect and trace deep oceanic circulation changes

holocene north atlantic overturning in an atmosphere ocean sea ice model compared to proxy based reconstructions climate and ocean circulation in the north atlantic region changed over the course of the holocene partly because of disintegrating ice sheets and partly because of an orbital induced insolation trend in the nordic seas this impact was accompanied by a rather small but significant amount of greenland ice sheet melting we have employed the emic loveclim and compared our model simulations with proxy based reconstructions of 13c sortable silt and magnetic susceptibility used to infer changes in past ocean circulation over the last 9000 years the various reconstructions exhibit different long term evolutions suggesting changes in either the overturning of the atlantic in total or of subcomponents of the ocean circulation such as the overflow waters across the greenland scotland ridge thus the question arises whether these reconstructions are consistent with each other or not a comparison with model results indicates that 13c employed as an indicator of overturning agrees well with the long term evolution of the modeled atlantic meridional overturning circulation amoc the model results suggest that different long term trends in subcomponents of the amoc such as iceland scotland overflow water are consistent with proxy based reconstructions and allow some of the reconstructions to be reconciled with the modeled and reconstructed from 13c amoc evolution we find a weak early holocene amoc which recovers by 7 kyr b p and shows a weak increasing trend of 88 1 msv kyr toward present with relatively low variability on centennial to millennial timescales

orbitally driven insolation forcing on holocene climate trends evidence from alkenone data and climate modeling a global spatial pattern of long term sea surface temperature sst trends over the last 7000 years is explored using a comparison of alkenone derived sst records with transient ensemble climate simulations with a coupled atmosphere ocean circulation model under orbitally driven insolation forcing the spatial trend pattern both in paleo sst data and in model results shows pronounced global heterogeneity generally the extratropics cooled while the tropics experienced a warming during the middle to late holocene we attribute these divergent holocene climate trends to seasonally opposing insolation changes furthermore climate mode changes similar to the arctic north atlantic oscillation are superimposed on the prevalent pattern it is concluded that nonlinear changes in the entire seasonal cycle of insolation played a dominant role for the temporal evolution of holocene surface temperatures for understanding of marine proxy data apart from the dominance of summer insolation in high latitudes a notable shift in the maximum insolation of the year in low latitudes has to be taken into account which may influence timing of phytoplankton production and thus alters the seasonal origin of temperature signals recorded in the proxies

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

circulation and deep water export at the western exit of the subpolar north atlantic the current system east of the grand banks was intensely observed by world ocean circulation experiment woce array acm 6 during 1993 95 with eight moorings reaching about 500 km out from the shelf edge and covering the water column from about 400 m depth to the bottom more recently a reduced array by the institut fur meerskunde ifm at kiel germany of four moorings was deployed during 1999 2001 focusing on the deep water flow near the western continental slope both sets of moored time series each about 22 months long are combined here for a mean current boundary section and both arrays are analyzed for the variability of currents and transports a mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper layer extrapolation and isopycnal subdivision of the mean transports into deep water classes the offshore part of the combined section is dominated by the deep reaching north atlantic current nac with currents still at 10 cm s 1 near the bottom and a total northward transport of about 140 sv sv 106 m3 s 1 with the details depending on the method of surface extrapolation used the mean flow along the western boundary was southward with the section mean north atlantic deep water outflow determined to be 12 sv below the 27 74 kg m 3 isopycnal however east of the deep western boundary current dwbc the deep nac carries a transport of 51 sv northward below 27 74 kg m 3 resulting in a large net northward flow in the western part of the basin from watermass signatures it is concluded that the deep nac is not a direct recirculation of dwbc water masses transport time series for the dwbc variability are derived for both arrays the variance is concentrated in the period range from 2 weeks to 2 months but there are also variations at interannual and longer periods with much of the dwbc variability being related to fluctuations and meandering of the nac a significant annual cycle is not recognizable in the combined current and transport time series of both arrays the moored array results are compared with other evidence on deep outflow and recirculation including recent models of different types and complexity

quantitative interpretation of atmospheric carbon records over the last glacial termination 1 the glacial interglacial rise in atmospheric pco2 is one of the best known changes in paleoclimate research yet the cause for it is still unknown forcing the coupled ocean atmosphere biosphere box model of the global carbon cycle bicycle with proxy data over the last glacial termination we are able to quantitatively reproduce transient variations in pco2 and its isotopic signatures 13c 14c observed in natural climate archives the sensitivity of the box model of the isotopic carbon cycle bicycle to high or low latitudinal changes is comparable to other multibox models or more complex ocean carbon cycle models respectively the processes considered here ranked by their contribution to the glacial interglacial rise in pco2 in decreasing order are the rise in southern ocean vertical mixing rates 30 ppmv decreases in alkalinity and carbon inventories 30 ppmv the reduction of the biological pump 20 ppmv the rise in ocean temperatures 15 20 ppmv the resumption of ocean circulation 15 20 ppmv and coral reef growth 5 ppmv the regrowth of the terrestrial biosphere sea level rise and the increase in gas exchange through reduced sea ice cover operate in the opposite direction decreasing pco2 during termination i by 30 ppmv according to our model the sequence of events during termination i might have been the following a reduction of aeolian iron fertilization in the southern ocean together with a breakdown in southern ocean stratification the latter caused by rapid sea ice retreat trigger the onset of the pco2 increase after these events the reduced north atlantic deep water nadw formation during the heinrich 1 event and the subsequent resumption of ocean circulation at the beginning of the bolling allerod warm interval are the main processes determining the atmospheric carbon records in the subsequent time period of termination i we further deduce that a complete shutdown of the nadw formation during the younger dryas was very unlikely changes in ocean temperature and the terrestrial carbon storage are the dominant processes explaining atmospheric 13c after the bolling allerod warm interval

glacial ocean overturning intensified by tidal mixing in a global circulation model due to lower sea levels during the last glacial maximum lgm tidal energy dissipation was shifted from the shallow margins into the deep ocean here using a high resolution tide model we estimate that global energy fluxes below 200 m depth were almost quadrupled during the lgm applying the energy fluxes to a consistent tidal mixing parameterization of a global climate model results in a large intensification of mixing global mean vertical diffusivity increases by more than a factor of 3 and consequently the simulated meridional overturning circulation accelerates by 21 46 in the model these effects are at least as important as those from changes in surface boundary conditions our findings contrast with the prevailing view that the abyssal lgm circulation was more sluggish we conclude that changes in tidal mixing are an important mechanism that may have strongly increased the glacial deep ocean circulation and should no longer be neglected in paleoclimate simulations

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

on the connection between dense water formation overturning and poleward heat transport in a convective basin an isopycnal two layer idealized model for a convective basin is proposed consisting of a convecting interior region and a surrounding boundary current buoyancy and wind driven parameterized eddy fluxes govern the exchange between the two to balance the interior buoyancy loss the boundary current becomes denser as it flows around the basin geostrophy imposes that this densification be accompanied by sinking in the boundary current and hence by an overturning circulation the poleward heat transport associated with convection in the basin can thus be viewed as a result of both an overturning and a horizontal circulation when adapted to the labrador sea the model is able to reproduce the bulk features of the mean state the seasonal cycle and even the shutdown of convection from 1969 to 1972 according to the model only 40 of the poleward heat buoyancy transport of the labrador sea is associated with the overturning circulation an exact solution is presented for the linearized equations when changes in the boundary current are small numerical solutions are calculated for variations in the amount of convection and for changes in the remotely forced circulation around the basin these results highlight how the overturning circulation is not simply related to the amount of dense water formed a speeding up of the circulation around the basin due to wind forcing for example will decrease the intensity of the overturning circulation while the dense water formation remains unvaried in general it is shown that the fraction of poleward buoyancy or heat transport carried by the overturning circulation is not an intrinsic property of the basin but can vary as a result of a number of factors

early holocene laurentide ice sheet deglaciation causes cooling in the high latitude southern hemisphere through oceanic teleconnection the impact of the early holocene laurentide ice sheet lis deglaciation on the climate at southern hemisphere high latitudes is studied in three transient simulations performed with a global climate model of the coupled atmosphere ocean vegetation system considering the lis deglaciation we quantify separately the impacts of the background meltwater fluxes and the changes in topography and surface albedo in our model the meltwater input into the north atlantic results in a substantial weakening of the atlantic meridional overturning circulation associated with absence of deep convection in the labrador sea northward ocean heat transport by the atlantic ocean is reduced by 28 this weakened ocean circulation leads to cooler north atlantic deep water nadw upwelling of this cool nadw in the southern ocean results in reduced surface temperatures by 1 degrees c to 2 degrees c here between 9 and 7 ka compared to an experiment without lis deglaciation poleward of the polar front zone this advective teleconnection between the southern and northern hemispheres overwhelms the effect of the classical bipolar seesaw mechanism these results provide an explanation for the relatively cold climatic conditions between 9 and 7 ka reconstructed in several proxy records from southern hemisphere high latitudes such as antarctic ice cores

interannual changes in the overflow from the nordic seas into the atlantic ocean through denmark strait the global thermohaline circulation is an important part of earth s climate system cold dense water formed in the nordic seas enters the atlantic ocean as overflows across the sills of the greenland scotland ridge the denmark strait overflow dso is one of the main sources of north atlantic deep water until now the dso has been believed to be stable on interannual timescales here for the first time evidence is presented from a 4 year program of observations showing that overflow transports in 1999 2000 were approximately 30 higher than previous estimates later transports decreased remarkably during the observation period coincident with a temporary temperature increase of about 0 5 c

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

contrasting trends in north atlantic deep water formation in the labrador sea and nordic seas during the holocene the holocene north atlantic deep water formation is studied in a 9 000 year long simulation with a coupled climate model of intermediate complexity forced by changes in orbital forcing and atmospheric trace gas concentrations during the experiment deep water formation in the nordic seas is reduced due to an enhanced influx of sea ice from the central arctic decreasing both surface salinity and density whereas deep water formation in the labrador sea increases due to surface cooling this leads to changes in the distribution of oceanic heat transported northwards by the atlantic ocean with less heat released 120 wm 2 in february in the nordic seas amplifying the surface cooling and increasing the sea ice cover in the labrador sea the oceanic heat release increases slightly 14 wm 2 thus dampening locally the cooling trend the overall atlantic overturning strength remains constant throughout the experiment over the nordic seas reduced evaporation contributes to the surface freshening

carbon isotope constraints on the deglacial co2 rise from ice cores the stable carbon isotope ratio of atmospheric co2 13catm is a key parameter in deciphering past carbon cycle changes here we present 13catm data for the past 24 000 years derived from three independent records from two antarctic ice cores we conclude that a pronounced 0 3 per mil decrease in 13catm during the early deglaciation can be best explained by upwelling of old carbon enriched waters in the southern ocean later in the deglaciation regrowth of the terrestrial biosphere changes in sea surface temperature and ocean circulation governed the 13catm evolution during the last glacial maximum 13catm and atmospheric co2 concentration were essentially constant which suggests that the carbon cycle was in dynamic equilibrium and that the net transfer of carbon to the deep ocean had occurred before then

influence of bering strait flow and north atlantic circulation on glacial sea level changes sea level fluctuations of about 20 30m occurred throughout the last glacial period these fluctuations seem to have been derived primarily from changes in the volume of northern hemisphere ice sheets 1 3 and cannot be attributed solely to ice melt caused by varying solar radiation 4 here we use a fully coupled climate model to show that the transport of relatively fresh pacific water into the north atlantic ocean was limited when lower sea level restricted or closed the bering strait resulting in saltier north atlantic surface waters this invigorated deep convection in the north atlantic ocean strengthening meridional overturning circulation and northward heat transport in our model which consequently promoted melting of ice sheets in north america and europe our simulations show that the associated sea level rise led to a reopening of the bering strait the flux of relatively fresh water into the north atlantic ocean muted meridional overturning circulation and led to cooling and ice sheet advance in the northern hemisphere we conclude that the repetition of this cycle could produce the sea level changes that have been observed throughout the last glacial cycle

the 8k event cause and consequences of a major holocene abrupt climate change abstract a prominent abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern hemisphere regions especially in wintertime in response to a very large outburst flood that freshened the north atlantic changes were much larger than typical climate variability before and after the event with anomalies up to many degrees contributing to major displacement of vegetative patterns this 8k event provides a clear case of cause and effect in the paleoclimatic realm and so offers an excellent opportunity for model testing the response to north atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following north atlantic freshening and so greatly strengthens the case that those older events also reflect north atlantic changes the north atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming caused ice melt and hydrologic cycle intensification at high latitudes lead to major changes in north atlantic circulation few model experiments have directly addressed the 8k event and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies so much work remains to be done

the spatial and temporal complexity of the holocene thermal maximum the holocene thermal maximum a period of relatively warm climate between 11 000 and 5 000 years ago1 2 is most clearly recorded in the middle and high latitudes2 3 of the northern hemisphere where it is generally associated with the local orbitally forced summer insolation maximum however proxy based reconstructions have shown that both the timing and magnitude of the warming vary substantially between different regions2 3 4 suggesting the involvement of extra feedbacks and forcings here we simulate the holocene thermal maximum in a coupled global ocean atmosphere vegetation model we find that before 7 000 years ago summers were substantially cooler over regions directly influenced by the presence of the laurentide ice sheet whereas other regions of the northern hemisphere were dominated by orbital forcing our simulations suggest that the cool conditions arose from a combination of the inhibition of labrador sea deep convection by the flux of meltwater from the ice sheet which weakened northward heat transport by the ocean and the high surface albedo of the ice sheet we thus conclude that interglacial climate is highly sensitive to relatively small changes in ice sheet configuration

two dimensional velocity structure beneath northern eurasia derived from the super long range seismic profile quartz abstract a two dimensional velocity structure for the mantle down to a depth of 700 km derived from super long range nuclear explosion seismic data recorded along the quartz profile in northern eurasia is presented the profile extends from the russian platform across western siberia to the altai mountains a detailed structural image of the lithosphere and asthenosphere shows strong lateral variations along the profile the subcrustal lithosphere to a depth of 100 km is characterized by lateral variations of the p wave velocity between 7 7 and 8 7 km sec the seismic data are consistent with the ural mountains having a crustal root another important finding in the data is an additional discontinuity in the mantle transition zone at 530 km this discontinuity could represent one or more phase transitions known to occur at about this depth at present the nuclear seismic records are believed to represent a key data set to prove the existence of this much debated upper mantle discontinuity

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

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

dynamics of north atlantic deep water masses during the holocene high resolution flow speed reconstructions of two core sites located on gardar drift in the northeast atlantic basin and orphan knoll in the northwest atlantic basin reveal a long term decrease in flow speed of northeast atlantic deep water neadw after 6 500 years benthic foraminiferal oxygen isotopes of sites currently bathed in neadw show a 0 2 depletion after 6 500 years shortly after the start of the development of a carbon isotope gradient between neadw and norwegian sea deep water we consider these changes in near bottom flow vigor and benthic foraminiferal isotope records to mark a significant reorganization of the holocene deep ocean circulation and attribute the changes to a weakening of neadw flow during the mid to late holocene that allowed the shoaling of lower deep water and deeper eastward advection of labrador sea water into the northeast atlantic basin

mantle heterogeneities and their significance results from lithoprobe seismic reflection and refraction wide angle reflection studiesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent lithoprobe contribution 1486 within lithoprobe s 10 transects data from more than 20 000 km of multichannel seismic mcs reflection profiling and 12 refraction wide angle reflection r war surveys were acquired while the

crust and upper mantle velocity structure of the intermontane belt southern canadian cordillera as part of the lithoprobe southern cordillera transect seismic refraction data were recorded along a 330 km long strike profile in the intermontane belt an iterative combination of two dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle p wave velocity structure this region is characterized by i a thin near surface layer with large variations in velocity between 2 8 and 5 4 km s and low velocity regions that correlate well with surface expressions of tertiary sedimentary and volcanic rocks ii an upper and middle crust with low average velocity gradient possibly a weak low velocity zone and lateral velocity variations between 6 0 and 6 4 km s iii a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth approximately 8 km thick with average velocities of 6 5 and 6 7 km s at top and base iv a depth to moho as defined by wide angle reflections that aver

spatial and temporal variability of holocene temperature in the north atlantic region the early holocene climate of the north atlantic region was influenced by two boundary conditions that were fundamentally different from the present the presence of the decaying laurentide ice sheet lis and higher than present summer solar insolation in order to assess spatial and temporal patterns of holocene climate evolution across this region we collated quantitative paleotemperature records at sub millennial resolution and synthesized their temporal variability using principal components analysis pca the analysis reveals considerable spatial variability most notably in the time transgressive expression of the holocene thermal maximum htm most of the region but especially areas peripheral to the labrador sea and hence closest to the locus of lis disintegration experienced maximum holocene temperatures that lagged peak summer insolation by 1000 3000 years many sites from the northeastern north atlantic sector including the nordic seas and scandinavia either warmed in phase with maximum summer insolation 11 000 9000 years ago or were less strongly lagged than the baffin bay labrador sea region these spatially complex patterns of holocene climate development which are defined by the pca resulted from the interplay between final decay of the lis and solar insolation forcing

laser vs settling velocity differences in silt grainsize measurements estimation of palaeocurrent vigour the use of grain size distribution of muds for the reconstruction of past deep ocean currents is becoming established and applied in the palaeoceanographic community the methods are also applicable to shallow marine and tidal flat muds with similar inferences concerning the energy of wave and current sorting being drawn fine sediment grain size distributions can be obtained using a variety of instruments based on fundamentally different theoretical principles these machines may give varying sometimes misleading results giving divergent interpretations of flow speed history the new evidence presented here combined with earlier work suggests that of the three most commonly used analytical methods for fine silts with clays settling velocity sedigraph should be the method and instrument of choice with electrical resistance pulse counters e g coulter counter as a suitable alternative the data also show that laser particle sizers should be avoided for palaeocurrent reconstructions because the measured size of platey minerals can be dominated by their large projected area this causes them to be recorded as the same size as larger equant grains although they have much smaller settling velocity and were deposited in aggregates this produces results with a weaker relationship to the dynamics of deposition the central problem is thus that some coarse clay fine silt is recorded as medium to coarse silt the key size in the sortable silt mean size method of inferring changes in flow speed for coarse silts e g loess the laser gives more satisfactory results

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

large δ13c gradients in the preindustrial north atlantic revealed the carbon isotopic composition 13 c 12 c expressed as 13 c of fossil foraminifera is the primary tracer used to infer changes in past ocean ventilation and its variations are interpreted by using the modern oceanic 13 c distribution as a framework however the present ocean 13 c distribution is strongly overprinted by isotopically light anthropogenic carbon dioxide a correction for this oceanic c 13 suess effect in the north atlantic na shows that the pristine na 13 c distribution has a richer and more detailed structure that is more clearly related to water mass distributions our results revise some fundamental perceptions regarding glacial interglacial ocean 13 c differences and allow paleo 13 c variations to be understood within the context of modern climate variability

global characterization of the holocene thermal maximum we analyze the global variations in the timing and magnitude of the holocene thermal maximum htm and their dependence on various forcings in transient simulations covering the last 9000 years 9 ka performed with a global atmosphere ocean vegetation model in these experiments we consider the influence of variations in orbital parameters and atmospheric greenhouse gases and the early holocene deglaciation of the laurentide ice sheet lis considering the lis deglaciation we quantify separately the impacts of the background melt water fluxes and the changes in topography and surface albedo in the analysis we focus on the intensity of the maximum temperature deviation relative to the preindustrial level its timing in the holocene and the seasonal expression in the model the warmest htm conditions are found at high latitudes in both hemispheres reaching 5 c above the preindustrial level while the smallest htm signal is seen over tropical oceans less than 0 5 c this latitudinal contrast is mostly related to the nature of the orbitally forced insolation forcing which is also largest at high latitudes and further enhanced by the polar amplification the holocene timing of the htm is earliest before 8 ka bp in regions not affected by the remnant lis particularly nw north america e asia n africa n south america the middle east ne siberia and australia compared to the early holocene insolation maximum the htm was delayed by 2 3 ka over ne north america and regions directly downwind from the lis a similar delay is simulated over the southern ocean while an intermediate lag of about 1 ka is found over most other continents and oceans the seasonal timing of the htm over continents generally occurs in the same month as the maximum insolation anomaly whereas over oceans the htm is delayed by 2 3 months exceptions are the oceans covered by sea ice and north africa were additional feedbacks results in a different seasonal timing the simulated timing and magnitude of the htm are generally consistent with global proxy evidence with some notable exceptions in the mediterranean region sw north america and eastern eurasia 2012 elsevier ltd

upper mantle structure in the southeastern canadian cordillera a seismic broadside wide angle experiment in canada s southeastern cordillera has revealed an anomalous and unexpected seismic arrival analysis of the data shows that the arrival is unlikely to be a crustal or shallow mantle phase or multiple or converted waves the kinematic characteristics of this arrival dictate that it is a reflection from within or from the base of a low velocity zone 7 8 8 0 km s 1 situated at a depth of 59 to 62 km that we identify as the top of the asthenosphere this leads to the conclusion that the base of the lithosphere is at a depth of 50 2 km in the region the interpreted shallow asthenosphere bolsters earlier conclusions of a heat source at depth in the southeastern canadian cordillera the interpretation also shows that stratification in the upper mantle is resolvable by current wide angle seismic experiments

deep water formation the subpolar gyre and the meridional overturning circulation in the subpolar north atlantic on interannual to decadal times scales model simulations suggest a strong relationship between anomalies in the deep water formation rate the strength of the subpolar gyre and the meridional overturning circulation in the north atlantic whether this is valid can only be confirmed by continuous long observational time series several measurement components are already in place but crucial arrays to obtain time series of the meridional volume and heat transport in the subpolar north atlantic are still missing here we summarize the recent developments of the deep water formation rates and the subpolar gyre transports we discuss how existing observational components in the subpolar north atlantic could be supplemented to provide long term monitoring of the meridional heat and volume transport through a combined analysis of observations and model results the temporal and spatial scales that had to be covered with instruments are discussed together with the key regions with the highest variability in the velocity and temperature fields

crustal velocity structure of the omineca and intermontane belts southeastern canadian cordillera we provide information on the physical properties across two important morphogeological regions of the southeastern canadian cordillera the omineca and intermontane belts and tie together two previously interpreted wide angle seismic lines which have separately probed each zone along strike the data analyzed here line 7 of lithoprobe s 1990 southern cordillera refraction experiment in southern british columbia is along a 410 km profile from the fraser river to radium hot springs near the rocky mountain trench iterative forward and inverse modeling of the seismic travel times constrained by seismic amplitude characteristics and earlier cross line interpretations defines a thin crust with relatively low velocities the upper crust has an average thickness of 15 3 km and an average p wave velocity of less than 6 1 km s 1 with velocities at the bottom reaching 6 2 6 3 km s 1 it is further characterized by velocity gradients which decrease markedly with depth and by low velocity trends which correlate with the traces of crustal scale shear zones a high velocity region at the west end of the profile is coincident with a major uplift structure the midcrust has an average thickness of 9 2 km with velocities below 6 1 km s 1 except for the extreme west end of the profile this defines a midcrustal low velocity zone lvz in the omineca belt and partly in the intermontane belt the lower crust from 24 km to 34 km depth has velocities averaging 6 6 6 7 km s 1 higher velocities in the deep crust may mark the westward extent of north american cratonic rocks at the okanagan valley the upper mantle velocity has an average 8 0 km 5 increasing to 8 2 km s 1 at the maximum depth of ray penetration at 48 km the model presented here compliments well the earlier interpretations of vertical incidence seismic reflection profiles the velocity structure of the crust especially the lvz closely correlates with the regional isotherms while there seems to be no well defined velocity boundary between the intermontane and omineca belts there are distinct velocity differences between the intermontane and neighboring coast plutonic complex to the west the low velocity characteristics of the omineca and much of the intermontane belts may be a consequence of temperature control of the physical parameters of the crust the lithosphere in the southeastern canadian cordillera is clearly anomalous with respect to much of northwest north america and in terms of crustal thickness and velocity structure it is similar only to the basin and range province of the united states

the last glacial cycle transient simulations with an aogcm a number of transient climate runs simulating the last 120 kyr have been carried out using famous a fast atmosphere ocean general circulation model aogcm this is the first time such experiments have been done with a full aogcm providing a three dimensional simulation of both atmosphere and ocean over this period our simulation thus includes internally generated temporal variability over periods from days to millennia and physical detailed representations of important processes such as clouds and precipitation although the model is fast computational restrictions mean that the rate of change of the forcings has been increased by a factor of 10 making each experiment 12 kyr long atmospheric greenhouse gases ghgs northern hemisphere ice sheets and variations in solar radiation arising from changes in the earth s orbit are treated as forcing factors and are applied either separately or combined in different experiments the long term temperature changes on antarctica match well with reconstructions derived from ice core data as does variability on timescales longer than 10 kyr last glacial maximum lgm cooling on greenland is reasonably well simulated although our simulations which lack ice sheet meltwater forcing do not reproduce the abrupt millennial scale climate shifts seen in northern hemisphere climate proxies or their slower southern hemisphere counterparts the spatial pattern of sea surface cooling at the lgm matches proxy reconstructions reasonably well there is significant anti correlated variability in the strengths of the atlantic meridional overturning circulation amoc and the antarctic circumpolar current acc on timescales greater than 10 kyr in our experiments we find that ghg forcing weakens the amoc and strengthens the acc whilst the presence of northern hemisphere ice sheets strengthens the amoc and weakens the acc the structure of the amoc at the lgm is found to be sensitive to the details of the ice sheet reconstruction used the precessional component of the orbital forcing induces 20 kyr oscillations in the amoc and acc whose amplitude is mediated by changes in the eccentricity of the earth s orbit these forcing influences combine to first order in a linear fashion to produce the mean climate and ocean variability seen in the run with all forcings

changes in the strength of the nordic seas overflows over the past 3000 years the nordic seas overflows constitute the densest component of the deep limb of the atlantic meridional overturning circulation amoc changes in the vigour of the overflows may have had important climatic effects in the past and may also have in the future yet evidence for multidecadal to millennial changes in the deep limb of the amoc and their potential relationship to north atlantic climate variability during the holocene remains weakly constrained here we present grain size data as a proxy for near bottom current speed from sub decadal to decadally resolved sediment cores located in the direct pathway of the two nordic overflows east and west of iceland the iceland scotland overflow water isow and the denmark strait overflow water dsow respectively the results show no clear relationship between reconstructed changes in the vigour of the nordic overflows and the well known periods of centennial scale climate variability recorded in the north atlantic region however well defined millennial scale trends are found in both of the overflow strength records over the last 3000 years which were possibly related to hydrographic reorganizations in the nordic seas driven by the decrease in northern hemisphere summer insolation over the neoglacial period a comparison between the near bottom flow speed reconstructions from isow and dsow suggests an anti phased relationship between the nordic seas overflows east and west of iceland over the last 3000 years this feature has been observed in climate models potentially as a result of shifts in the deep water formation sites as a response to changes in atmospheric patterns over the nordic seas

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

observed interannual variability of the atlantic meridional overturning circulation at 26 5 n the atlantic meridional overturning circulation moc plays a critical role in the climate system and is responsible for much of the heat transported by the ocean a mooring array nomianally at 26 circ n between the bahamas and the canary islands deployed in apr 2004 provides continuous measurements of the strength and variability of this circulation with seven full years of measurements we now examine the interannual variability of the moc while earlier results highlighted substantial seasonal and shorter timescale variability there had not been significant interannual variability the mean moc from 1 apr 2004 to the 31 march 2009 was 18 5 sv with the annual means having a standard deviation of only 1 0 sv from 1 april 2009 to 31 march 2010 the annually averaged moc strength was just 12 8 sv representing a 30 decline this downturn persisted from early 2009 to mid 2010 we show that the cause of the decline was not only an anomalous wind driven event from dec 2009 mar 2010 but also a strengthening of the geostrophic flow in particular the southward flow in the top 1100 m intensified while the deep southward return transport particularly in the deepest layer from 3000 5000 m weakened this rebalancing of the transport from the deep overturning to the upper gyre has implications for the heat transported by the atlantic

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

holocene evolution of deep circulation in the northern north atlantic traced by sm nd and pb isotopes and bulk sediment mineralogy 1 bulk mineralogy sm nd and pb elemental and isotopic compositions of the clay size fraction of holocene sediments were analyzed in three deep north atlantic cores to trace the particle provenance the aims of the present paper are to identify the origin of the particles driven by deep currents and to reconstruct deep circulation changes over the holocene in the north atlantic the three cores are retrieved in fracture zones two of them are located in the island basin along the gyre of north atlantic deep water and the third core is located off the present deep circulation gyre in the labrador sea whereas sedimentary supplies in the labrador sea were constantly derived from proximal sources the geochemical mixing trends in the iceland basin samples indicate pronounced changes in the relative contribution of continental margin inputs over the past 6 kyr supplies from western european margin that sharply increased at 6 kyr were progressively diluted by a larger contribution of scandinavian margins over the last 3 kyr changes in composition of the particles imply significant reorganization of paleocirculation of the deep north atlantic components in the eastern basins mainly reorganizations for both iceland scotland overflow water and norwegian sea overflow water moreover the unusual pb isotopic composition of the oldest sediments from the southern iceland basin indicates that distal supplies from greenland margin were driven into the iceland basin supporting a deep connection between labrador sea and iceland basin through the charlie gibbs fracture zone prior the holocene transition period

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

variations in the strength of the north atlantic bottom water during holocene abstract we report here on a multi proxy study of the changes in the dynamics and the properties of bottom water mass in the subpolar north altantic during holocene magnetic properties coupled with sortable silt and benthic carbon isotopes are investigated for holocene marine sedimentary sequences located in the charlie gibbs fracture zone 53 n and in central 57 n and southern gardar drift 59 n all the cores are located at water depths bathed by the iceland scotland overflow water isow mixed at the southernmost locality with southern sourced water masses the long term variations in measured proxies are fitted with similar polynomial curves an early holocene event characterized by a shutdown shoaling of the bottom circulation at the deepest sites is most likely related to the main deglacial freshwater inputs it is followed by a progressive strengthening deepening of the overflow water which culminates around 6 kyr in coincidence with the holocene thermal maximum after 6 kyr corresponding to a drastic hydrological reorganization in the north atlantic a general decline in the bottom flow strength is observed until about 2 kyr b p when it reached its present day state after detrending several short periods of reduced bottom flow strength and sedimentary transport from the northern detrital sources are observed with a periodicity of around 600 yr with no clear relationship at this time scale between surface and deep ocean

palaeo oceanography deepwater variability in the holocene epoch the conversion of surface water to deep water in the north atlantic results in the release of heat from the ocean to the atmosphere which may have amplified millennial scale climate variability during glacial times1 and could even have contributed to the past 11 700 years of relatively mild climate known as the holocene epoch 2 3 4 here we investigate changes in the carbon isotope composition of benthic foraminifera throughout the holocene and find that deep water production varied on a centennial millennial timescale these variations may be linked to surface and atmospheric events that hint at a contribution to climate change over this period