the hafnium neodymium isotopic composition of atlantic seawater the first full water column hafnium isotopic compositions of atlantic seawater have been obtained at seven locations from the labrador sea to the drake passage despite subpicomolar concentrations in seawater a precision of the hf isotopic measurements of 03 epsilon hf units was achieved an overall epsilon hf range between 3 1 in the labrador sea and 4 4 in antarctic bottom water was determined the distribution of which broadly reflects continental weathering inputs within particular water column profiles significant differences of up to 4 epsilon hf units occur combined with nd isotope data of the same samples it is evident that the hf isotopic composition of seawater is too radiogenic for a given nd isotopic composition and that the largest difference between expected and measured hf isotopic compositions in seawater occurs near the oldest continental crust in the labrador sea this corroborates the previous proposition which was mainly based on ferromanganese crust data that the hf isotopic composition of seawater is controlled by incongruent weathering of continental crust and possibly to some extent by hydrothermal contributions hafnium concentrations in the ocean do not increase along the deep ocean conveyer indicating an oceanic residence time of only a few hundred years which is significantly shorter than previously assumed the hf isotopic composition of past seawater can therefore serve as a proxy for short distance basin scale mixing processes and the regime and intensity of nearby continental weathering processes c 2009 elsevier b v all rights reserved

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

microfabric and microshear evolution in deformed till abstract the bed deformation model of glacier movement requires that ice masses shear their beds to high strains for homogeneous basal tills that can result from such deformation however there are no sedimentological criteria for estimating strain magnitude thus a large ring shear device was used to shear two basal tills and their particle fabrics and microshears were studied at various strains up to 108 sand particle fabrics in a longitudinal flow plane became steady at shear strains of 7 39 steady fabrics were moderately strong with s1 eigenvalues of 0 71 0 74 these values are smaller than those from experiments with pebble sized particles perhaps reflecting the greater probability of sand particles colliding with larger grains fabrics parallel to flow strengthened progressively indicating that particles did not reorient with their long axes transverse to flow r1 and r2 microshears developed and persisted to the highest strains attained r1 shears became more pervasive with strain and oriented closer to the shear plane movement along both sets of shears provides an explanation for the plunge of the steady state sand fabric approximately 10 upglacier these experiments provide a basis for estimating till deformation magnitude but results will require cautious application to the geologic record where deformation processes may be ambiguous

asynchronous little ice age glacier fluctuations in iceland and european alps linked to shifts in subpolar north atlantic circulation abstract records of past glacier fluctuations are an important source of paleoclimate data and provide context for future changes in global ice volume in the north atlantic region glacier chronologies can be used to track the response of terrestrial environments to variations in marine conditions including circulation patterns and sea ice cover however the majority of glacier records are discontinuous and temporally restricted owing in part to the extensive advance of northern hemisphere glaciers during the little ice age lia the most recent and severe climate anomaly of the neoglacial period here we combine an absolutely dated and continuous record of langjokull ice marginal fluctuations with new reconstructions of sediment flux through the past 1 2 ka using varved sediments from hvitarvatn a proglacial lake in iceland s central highlands large spatial and temporal variations in sediment flux related to changing ice cap dimensions are reconstructed from six sediment cores and seismic reflection profiles sediment data reveal two discrete phases of ice expansion occurring ca 1400 to 1550 ad and ca 1680 to 1890 ad these advances are separated by a persistent interval of ice retreat suggesting that a substantial period of warming interrupted lia cold the pattern of icelandic glacier activity contrasts with that of european glaciers but shows strong similarities to reconstructed changes in north atlantic oceanographic conditions indicating differing regional responses to coupled ocean atmosphere sea ice variations our data suggest that subpolar north atlantic circulation dynamics may have led to coherent asynchronous glacier fluctuations during the mid lia and highlight the importance of circulation variability in triggering and transmitting multidecadal scale climate changes to nearby terrestrial environments

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

late holocene intermediate water variability in the northeastern atlantic as recorded by deep sea corals abstract the nd isotopic composition of the aragonite skeleton of fossil deep sea corals lophelia pertusa madrepora oculata and desmophyllum dianthus located in the northeastern atlantic at water depths between 635 and 1300 m was investigated to reconstruct changes in the atlantic mid depth gyre circulation during the past millennium the coral end values varied systematically from 11 8 to 14 4 during the past 1500 years reflecting variations in seawater end and thus water mass provenance low end values end 14 occurred during the warm medieval climatic anomaly mca between 1000 ad and 1250 ad and during the most recent period 1950 ad to 2000 ad interrupted by a period of significantly higher end values 12 5 during the little ice age lia between 1350 ad and 1850 ad one long lived branching coral even recorded an abrupt systematic rise from low to high end values around 1250 ad over the course of its 10 year growth period these variations are interpreted to result from variable contributions of the subpolar and subtropical atlantic intermediate water masses which today are characterized by end values of 15 and 11 respectively the low end values observed during the warm mca and during recent times imply a strong eastward extension of the mid depth subpolar gyre spg induced by a dominant positive phase of the north atlantic oscillation nao during the lia water from the subtropical gyre stg and potentially from the mediterranean sea water msw propagated further northward as indicated by the higher coral end values this pattern suggests a negative mean state of the nao during the lia with weaker and more southerly located westerlies and a westward contraction of the spg variations in the contributions of the two gyres imply changes in the heat and salt budgets at intermediate depths during the past millennia that may have contributed to changes in the properties of north atlantic inflow into the nordic seas and thus deep water formation

high resolution temperature observations of a trapped nonlinear diurnal tide influencing cold water corals on the logachev mounds abstract a high resolution thermistor string mooring of 120 m length was used to measure turbulence processes in the water layer above the foot of a densely populated cold water coral mound at 919 m depth at the southeast slope of rockall bank in the logachev area north east atlantic ocean as expected from previous current observations the temperature data reveal a dominant diurnal tidal periodicity associated with topography trapped weakly bottom intensified waves these baroclinic diurnal waves are driven to near resonance around the mound and have vertical amplitudes exceeding the mooring line their horizontal excursion length matches the size of the mound which causes a residual current around and flux up the mound as their particle velocities also match the phase speed of the wave traveling around the mound these waves can become highly nonlinear and show largest turbulence due to wave breaking at the transition from warming downslope to cooling upslope tidal phase averaged over the entire 9 day time series and the 120 m vertical range mean turbulent kinetic energy dissipation amounts to 2 2 1 1 10 7 w kg 1 and mean vertical eddy diffusivity to 9 5 10 3 m2 s 1 with short term variations over four orders of magnitude such large turbulence more than 100 times larger than open ocean values and comparable with that observed in tidally energetic shelf break shallow sea areas will affect the nutrient replenishment of the cold water corals

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

warm and saline events embedded in the meridional circulation of the northern north atlantic 1 ocean state estimates from 1958 to 2005 from the simple ocean assimilation system soda system are analyzed to understand circulation between subtropical and subpolar atlantic and their connection with atmospheric forcing this analysis shows three periods 1960s around 1980 and 2000s with enhanced warm saline waters reaching high latitudes alternating with freshwater events originating at high latitudes it complements surface drifter and altimetry data showing the subtropical subpolar exchange leading to a significant temperature and salinity increase in the northeast atlantic after 2001 the warm water limb of the atlantic meridional overturning cell represented by soda expanded in density salinity space during these warm events tracer simulations using soda velocities also show decadal variation of the gulf stream waters reaching the subpolar gyre and nordic seas the negative phase of the north atlantic oscillation index usually invoked in such variability fails to predict the warming and salinization in the early 2000s with salinities not seen since the 1960s wind stress curl variability provided a linkage to this subtropical subpolar gyre exchange as illustrated using an idealized two layer circulation model the ocean response to the modulation of the climatological wind stress curl pattern was found to be such that the northward penetration of subtropical tracers is enhanced when amplitude of the wind stress curl is weaker than normal in this case both the subtropical and subpolar gyres weaken and the subpolar density surfaces relax hence the polar front moves westward opening an enhanced northward access of the subtropical waters in the eastern boundary current

impact of freshwater release in the north atlantic under different climate conditions in an oagcm abstract the response of climate to freshwater input in the north atlantic na has raised a lot of concern about the issue of climate stability since the discovery of abrupt coolings during the last glacial period such coolings have usually been related to a weakening of the atlantic meridional overturning circulation amoc probably associated with massive iceberg surges or meltwater pulses additionally the recent increase in greenhouse gases in the atmosphere has also raised the possibility of a melting of the greenland ice sheet which may impact the future amoc and thereby the climate in this study the extent to which the mean climate influences the freshwater release linked to ice sheet melting in the na and the associated climatic response is explored for this purpose the simulations of several climatic states last interglacial last glacial maximum mid holocene preindustrial and future 2 co2 are considered and the climatic response to a freshwater input computed interactively acc

deep sea coral evidence for rapid change in ventilation of the deep north atlantic 15 400 years ago coupled radiocarbon and thorium 230 dates from benthic coral species reveal that the ventilation rate of the north atlantic upper deep water varied greatly during the last deglaciation radiocarbon ages in several corals of the same age 15 41 0 17 thousand years and nearly the same depth 1800 meters in the western north atlantic ocean increased by as much as 670 years during the 30 to 160 year life spans of the samples cadmium calcium ratios in one coral imply that the nutrient content of these deep waters also increased our data show that the deep ocean changed on decadal centennial time scales during rapid changes in the surface ocean and the atmosphere

apparent long term cooling of the sea surface in the northeast atlantic and mediterranean during the holocene reconstructions of upper ocean temperature t during the holocene 10 0 ka b p were established using the alkenone method from seven high accumulation sediment cores raised from the northeast atlantic and the mediterranean sea 361n 751n all these paleo t records document an apparent long term cooling during the last 10 kyr in records with indication of a constant trend the apparent cooling ranges from 0 27 to 0 151c kyr 1 records with indication of time variable trend show peak to peak amplitudes in apparent temperatures of 1 2 2 91c a principal component analysis shows that there is one factor which accounts for a very large fraction 67 of the total variance in the biomarker paleo t records and which dominates these records over other potential secondary influences two possible contributions are 1 a widespread surface cooling which may be associated with the transition fromthe hypsithermal interval b9 5 7 ka b p to the neoglaciation b5 7 0 ka b p and 2 a change in the seasonal timing and or duration of the growth period of alkenone producers prymnesiophyte algae the first contribution is consistent with many climate proxy records from the northeast atlantic area and with climate model simulations including milankovitch forcing the second contribution is consistent with the divergence between biomarker and summer faunal paleo t fromearly to late holocene observed in two cores further work is necessary and in particular the apparent discordance between biomarker and faunal t records for the relative stable holocene period must be understood to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records r 2002 elsevier science ltd all rights reserved

the importance of the permanent thermocline to the cold water coral carbonate mound distribution in the ne atlantic abstract a prominent feature of the nw european continental slope is the presence of numerous cold water coral carbonate mounds that are clustered in a number of provinces these provinces occupy a relatively narrow depth range along the continental slope 95 of all coral carbonate mounds identified on the irish seabed have their mound bases between 500 and 1000 m water depths with a peak in distribution at 650 m water depth the distribution in mound base depths is skewed with a tail extending from the maximum at 650 m to deeper depths this distribution brackets the depth of the permanent thermocline in the ne atlantic 600 1000 m formed below the base of the winter mixed layer it is shown that the permanent thermocline is associated with the strongest residual near seabed current flow with typical residual current speeds up to 2 3 times larger at the thermocline depth compared to other depths the strong vertical density gradient associated with the permanent thermocline together with the steep continental slope at those depths also enhances the energy of certain periodic motions such as internal waves and baroclinic tidal currents these dynamic conditions favour mound growth through the promotion of significant along slope sediment transport and also provide large across slope sediment movement and organic matter fluxes the stability of the thermocline structure is likely the key in providing favourable conditions over long time scales that allow mound growth through sediment baffling processes

acquisition of the neodymium isotopic composition of the north atlantic deep water the north atlantic deep water nadw neodymium isotopic composition nd ic is increasingly used in oceanography and paleoceanography to trace large scale circulation and weathering processes notably to investigate past variations of the global thermohaline circulation although the present day nadw nd ic is well characterized at nd 13 5 the acquisition of this isotopic signature in other words the causes of this value has so far been very sparsely documented such an understanding is however fundamental to the interpretation of paleo records nd ic and rare earth element concentrations were measured at 9 stations within the north atlantic subpolar gyre signature cruise summer 1999 the comparison of this data set with our understanding of water mass circulation provides a description of how the three layers constituting the nadw the labrador sea water lsw nd 13 9 0 4 north east atlantic deep water neadw nd 13 2 0 4 and north west atlantic bottom water nwabw nd 14 5 0 4 acquire their nd ic through distinct water mass mixings and lithogenic inputs these different mechanisms acting upon water masses from very diverse sources seem to bring the nd ic of the three nadw layers to values close together and similar to that of the nadw it is suggested that sediment seawater interactions significantly lower the neadw and nwabw nd ic along the south east greenland margin since these interactions do not significantly modify the nd content of these water masses sediment remobilizations leading to the nd ic variations are probably associated with nd removal fluxes from the water mass toward the sediment a process called boundary exchange on the other hand lsw seems to acquire its nd ic from the subpolar mode waters from which it is formed by deep convection and no other mechanism needs to be invoked its unradiogenic signature could ultimately be linked to fresh water runoff from the canadian shield these conclusions should allow more precise interpretations of paleoceanographic nd ic records taking into account the distinct histories of the three nadw layers including distinct water mass mixings and distinct lithogenic inputs

variability of the oceanic mixed layer 1960 2004 abstract a new monthly uniformly gridded analysis of mixed layer properties based on the world ocean atlas 2005 global ocean dataset is used to examine interannual and longer changes in mixed layer properties during the 45 yr period 1960 2004 the analysis reveals substantial variability in the winter spring depth of the mixed layer in the subtropics and midlatitudes in the north pacific an empirical orthogonal function analysis shows a pattern of mixed layer depth variability peaking in the central subtropics this pattern occurs coincident with intensification of local surface winds and may be responsible for the sst changes associated with the pacific decadal oscillation years with deep winter spring mixed layers coincide with years in which winter spring sst is low in the north atlantic a pattern of winter spring mixed layer depth variability occurs that is not so obviously connected to local changes in winds or sst suggesting that other processes such as advection are more important interestingl

millennial scale variations of the holocene north atlantic mid depth gyre inferred from radiocarbon and neodymium isotopes in cold water corals abstract variations in north atlantic ocean mid depth circulation during the holocene are poorly understood it is believed that they had a significant influence on the properties of water entering the nordic sea by redistributing heat and freshwater potentially affecting deep water formation and climate to improve our knowledge of the ne atlantic mid depth circulation radiocarbon and neodymium isotope analyses have been then carried out on precisely dated u th l pertusa and m oculata coral fragments from two sediment cores taken at 750 m water depth on the sw rockall trough margin cold water coral cwc end values vary between 12 2 0 3 and 16 6 0 4 and result from variable contributions of unradiogenic mid depth subpolar gyre mid spg water 15 and more radiogenic eastern north atlantic water enaw 11 which is transported northward to the rockall trough by boundary currents along the european margin increased coral end reflects a westward contraction of the mid spg water and a higher proportion of enaw the mid holocene from 8 8 to 6 8 ka bp is marked by unradiogenic coral end from 16 6 0 4 to 14 6 0 5 indicating a greater eastward extension of the mid spg this is followed by a shift from 6 8 to 5 ka bp toward more radiogenic end values from 15 4 0 3 to 13 3 0 2 suggesting a westward contraction of the mid spg and a higher proportion of enaw the mid holocene long term change in end is characterized by millennial variations of up to 2 5 epsilon units well marked during the late holocene indicating that eastward extension of the mid spg coeval with warm periods in northern europe e g the medieval climatic anomaly and the roman warm period most of the cwc derived 14c values match the global ocean values indicating that the water masses bathing the corals were generally well ventilated the exceptions are a few short intervals of eastward extension of the mid spg which are characterized by lower 14c during the late holocene we propose that these minor 14c fluctuations in the rockall trough may be related to local changes in the mixed layer depth or to variability in the advection of water from the labrador sea where deep convection gives rise to thermocline waters that are relatively depleted in terms of 14c the eastward extension of the mid spg between 8 8 and 6 8 ka bp is associated with the holocene climatic optimum concurrent with a maximum strength of the iceland scotland overflow water isow which is indicative of an increase in the atlantic meridional overturning circulation amoc this period is followed by a transition in the north atlantic circulation which occurred around 6 8 ka bp and coincides with the onset of labrador sea water formation a decrease of the isow strength and a contraction of the mid spg leading to a greater intrusion of saline subtropical water into the subpolar atlantic

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

amplitude and timing of temperature and salinity variability in the subpolar north atlantic over the past 10 k y paired planktic foraminiferal 18 o and mg ca data reveal trends of increasing temperatures 3 c and salinities in the subpolar north atlantic over the course of the holocene which were punctuated by abrupt events the trends likely refl ect an insolation forced northward retreat of the boundary between polar and north atlantic subsurface waters the superimposed variability does not appear to be periodic but tends to recur within a broad millennial band the records provide convincing evidence of open ocean cooling nearly 2 c and freshening during the 8 2 ka event and suggest similar conditions at 9 3 ka however the two largest temperature oscillations in our record 2 c occurred during the past 4 k y suggesting a recent increase in temperature variability relative to the mid holocene perhaps in response to neoglaciation which began at about this time

the climate influence on the mid depth northeast atlantic gyres viewed by cold water corals 1 the neodymium nd isotopic composition expressed in epsilon units nd of reef framework forming cold water corals provides unique measures of water mass provenance and mixing within the northeast atlantic today and in the past a reconstruction of near thermocline water nd from cold water corals of the gulf of cadiz and porcupine seabight spanning over the past 300 000 years now revealed that climate cooling during marine isotope stages mis 7 2 and mis 8 9 led to a retraction of the mid depth subpolar gyre mspg to the west conversely northern hemisphere warming and increasing fresh water fluxes to the northwest labrador sea favor a stronger eastward extension of the mspg blocking the northward flow of temperate atlantic water as observed during the early mis 1 and the early stage mis 5 5 these changes are likely the result of large scale south north displacement of the westerlies similar to present day observations that the north atlantic oscillation nao is linked with mid depth ocean circulation based on these observations we hypothesize that further climate warming will also strengthen the mspg leading to a salt and temperature decrease in the northeast atlantic whereas salinity and temperature will increase in the temperate atlantic however the amplitude of such changes on north atlantic overturning remains to be tested

coupling of palaeoceanographic shifts and changes in marine reservoir ages off north iceland through the last millennium abstract our study of spatial and temporal marine reservoir age changes on the north icelandic shelf is based on two high resolution 1000 year sedimentary records from images core sites md99 2273 and md99 2275 located at 650 m and 470 water depths respectively the sedimentary and fossil record is extremely sensitive to past oceanographic and climatic changes in this region it has been demonstrated that the position of the polar front across the shelf has been very dynamic through the last millennium and our results show that deviations in the marine reservoir ages can be used as a palaeoceanographic tool for tracing changes in the water masses the presence of historically and terrestrially dated tephra markers from icelandic volcanoes makes it possible to construct a reliable tephrochronological age depth model for each of the marine sediment cores a comparison of these age models with age models based on calibrated ams radiocarbon age determinations of marine molluscs displays major deviations through the last 1000 years the discrepancy between the two types of age models shows that there was an average reservoir age of about 650 years at md99 2273 and 590 years at site md99 2275 with considerable changes through time this is suggested to be related to a generally high but fluctuating inflow of apparently old polar waters to the area a comparison with the distribution of two benthic foraminiferal indicator species in the cores one for polar and arctic water masses islandiella norcrossi and one for atlantic water cassidulina neoteretis as well as with different proxies for sea ice cover in the area shows that there was an increase in the penetration of cold polar and arctic water masses after ad 1300 i e during the little ice age at both sites resulting in an increase in the reservoir ages relatively high reservoir ages throughout the last millennium at the deepest core site md99 2273 can be attributed to a continuously high polar water influence at the sea floor the location of this site to the west of the kolbeinsey ridge which has acted as a submarine barrier throughout the holocene may have contributed to a relatively high influence of cold denmark strait overflow water in this area

vivaldi 1991 a study of the formation circulation and ventilation of eastern north atlantic central water abstract a synoptic hydrographic data set comprising 32 full depth ctd casts and 2500 ctd seasoar profiles to 500 m is used to describe the s properties and circulation of central water east of the mid atlantic ridge and between 39 n and 54 n eastward transport of 20 10 6 m 3 s 1 in the north atlantic current turns entirely northwards to the west of 54 n 20 w this transport consists in the upper layers of western north atlantic water freshened at temperatures below 10 c by mixing with subarctic intermediate water northern and southern branches of the north atlantic current are well defined and both turn northwards west of 20 w a further 10 10 6 m 3 s 1 of eastern north atlantic water forms and recirculates anticyclonically to the west of spain south of the north atlantic current and north of 40 n eastern north atlantic water is most weakly stratified east of 20 w and there is clear correlation between weakly stratified pycnostads and positive salinity anomalies relative to western north atlantic water thus eastern north atlantic water is a winter mode water in which strong winter cooling has increased the density and hence also the salinity anomaly at a given temperature near the southern entrance to the rockall trough there is evidence that salinities are also increased by mediterranean water influence circulation south of the north atlantic current is complex there is no evidence for direct ventilation southwards across 40 n where water properties s potential vorticity and cfc 113 and historical data all indicate westward ventilation east of 24 w with weak southward ventilation occurring further west in the vicinity of the azores the circulation pattern suggested is remarkably similar to that proposed by helland hansen and nansen in 1926 the eastern north atlantic geophysiske publicajoner 4 1 76 with anticyclonic circulation of colder eastern north atlantic water north of 40 n meeting warmer water from south of 40 n circulating cyclonically north of the azores current the distribution of pycnostads and s properties between 20 w and 35 w north of the azores indicates alternate bands of western and eastern north atlantic water moving eastward and westward respectively including evidence for westward motion immediately south of the southern branch of the north atlantic current possibly by westward propagation of anticyclonic eddies containing deep pycnostads

structure and development of giant carbonate mounds at the sw and se rockall trough margins ne atlantic ocean abstract high resolution seismic reflection profiling carried out in 1997 1999 showed that giant carbonate mounds occur between 500 and 1200 m water depth along both the se and sw margins of rockall trough the mounds rise 5 300 m above the surrounding seafloor and have diameters at their bases of up to 5 km buried mounds at relatively shallow depths below the seafloor are also found both individual and complex clusters of mounds can be recognized smaller and individual sometimes buried mounds are found at the upper slope on the sw rockall trough margin higher steeper and individual mounds are found deeper downslope 900 1100 m at the middle slope the mounds merge into a complex structure and form complex clusters with a very irregular upper surface and an apparent lack of internal reflectors 600 1000 m depth initial results of high resolution 3d seismic profiling at 30 m line spacing in august 2000 in two boxes of 750 m 14 5 km covering a small central part of the sw rockall trough mound area indicate two stages of mound development here the latest probably of pliocene holocene age this age based on the presence of a regionally recognizable unconformity below the mounds is also assumed for the mounds of the se rockall trough porcupine bank margin

multicentennial variability of the sea surface temperature gradient across the subpolar north atlantic over the last 2 8 kyr a 2800 year long august sea surface temperature asst record based on fossil diatom assemblages is generated from a marine sediment core from the northern subpolar north atlantic the record is compared with the asst record from the norwegian sea to explore the variability of the asst gradient between these areas during the late holocene the asst records demonstrate the opposite climate tendencies towards a persistent warming in the core site in the subpolar north atlantic and cooling in the norwegian sea at the multicentennial scale of asst variability of 600 900 years the records are nearly in anti phase with warmer colder periods in the subpolar north atlantic corresponding to the colder warmer periods in the norwegian sea at the shorter time scale of 200 450 years the records display a phase locked behaviour with a tendency for the positive asst anomalies in the norwegian sea to lead by 30 years the negative asst anomalies in the subpolar north atlantic this apparent asst seesaw might have an effect on two major anomalies of the european climate of the past millennium medieval warm period mwp and the little ice age lia during the mwp warming of the sea surface in the norwegian sea occurred in parallel with cooling in the northern subpolar north atlantic whereas the opposite pattern emerged during the lia the results suggest that the observed asst seesaw between the subpolar north atlantic and the norwegian sea could be a surface expression of the variability of the eastern and western branches of the atlantic meridional circulation amoc with a possible amplification through atmospheric feedback

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

the north atlantic oscillation surface current velocities and sst changes in the subpolar north atlantic changes in surface circulation in the subpolar north atlantic are documented for the recent interannual switch in the north atlantic oscillation nao index from positive values in the early 1990s to negative values in 1995 96 data from lagrangian drifters which were deployed in the north atlantic from 1992 to 1998 were used to compute the mean and varying surface currents ncep winds were used to calculate the ekman component allowing isolation of the geostrophic currents the mean ekman velocities are considerably smaller than the mean total velocities that resemble historical analyses the northeastward flow of the north atlantic current is organized into three strong cores associated with topography along the eastern boundary in rockall trough in the iceland basin the subpolar front and on the western flank of the reykjanes ridge irminger current the last is isolated in this eulerian mean from the rest of the north atlantic current by a region of weak velocities on the east side of the reykjanes ridge the drifter results during the two different nao periods are compared with geostrophic flow changes calculated from the nasa pathfinder monthly gridded sea surface height ssh variability products and the advanced very high resolution radiometer avhrr sst data during the positive nao years the northeastward flow in the north atlantic current appeared stronger and the circulation in the cyclonic gyre in the irminger basin became more intense this was consistent with the geostrophic velocities calculated from altimetry data and surface temperature changes from avhrr sst data which show that during the positive nao years with stronger westerlies the subpolar front was sharper and located farther east sst gradients intensified in the north atlantic current irminger basin and east of the shetland islands during the positive nao phase associated with stronger currents sst differences between positive and negative nao years were consistent with changes in air sea heat flux and the eastward shift of the subpolar front sst advection as diagnosed from the drifters likely acted to reduce the sst differences

nd isotopic composition of water masses and dilution of the mediterranean outflow along the southwest european margin nd isotopic compositions epsilon nd of seawater profiles and deep sea corals collected off the coast of iberia and from the bay of biscay were measured 1 to constrain the nd isotopic composition of water masses along the southwest european margin 2 to track the mediterranean outflow water mow during its northward propagation and 3 to establish hydrological changes during the last 1500 years the eastern north atlantic central water enacw is characterized by nd isotopic composition of around 12 0 mediterranean sea water msw is collected from 800 and 1200 m depth and is characterized by epsilon nd values ranging from 10 9 off the coast of iberia to 11 6 in the bay of biscay these epsilon nd results suggest a strong dilution of the pure mow at the strait of gibraltar epsilon nd 9 4 of approximately 40 and 30 along its northward circulation pathway essentially with a contribution from enacw at around 2000 m depth epsilon nd water profiles display the occurrence of a nonradiogenic water mass epsilon nd 13 originating from the labrador sea labrador sea water fossil deep sea corals dated between 84 and 1500 years display nd isotopic compositions that vary moderately from present day seawater values suggesting a weaker influence of mow in the formation of msw during the dark ages and the little ice age these recent cold events seem to be associated with a reduction in the northward penetration of msw which may result from a greater eastward extension of the middepth subpolar gyre and or a reduction of msw formation likely tied to a variation in deep mediterranean water production

north atlantic oscillation induced changes of the upper layer circulation in the northern north atlantic ocean 1 hydrographic data of the 1990s along world ocean circulation experiment sections a1e greenland ireland and a2 newfoundland france indicate a redistribution of cold low saline subarctic waters and warm saline subtropical waters in the upper layer of the northern north atlantic within about 2 years after the north atlantic oscillation had turned from a period with strong westerlies until 1995 to a period with weak westerlies in 1996 and 1997 in the latter period subarctic waters spread preferably southward along the continental slope east of newfoundland rather than eastward with the north atlantic current nac into the irminger iceland and west european basins as a consequence the subarctic front and the associated nac shifted eastward in the newfoundland basin where subarctic waters accumulate and westward in the iceland basin where a large warm and saline anomaly was found in 1996 and 1997 indicating a contraction of the subpolar gyre in the irminger basin the anomaly occurred in 1998 and 1999 whereas the cyclonic circulation in the irminger and iceland basins weakened it intensified in the newfoundland basin where a strengthened labrador current retroflection joined the nac this and the increased anticyclonic recirculation of subtropical waters in the newfoundland and west european basins caused a considerable reduction of the northward heat transport across 47 n in the upper layer in 1997 the anticyclonic circulation occurring in the west european basin suggested a northward expansion of the subtropical gyre

evolution of subpolar north atlantic surface circulation since the early holocene inferred from planktic foraminifera faunal and stable isotope records abstract past changes in the surface flow regime of two main eastern north atlantic warm water pathways toward the nordic seas were reconstructed based on faunal analyses in combination with carbon and oxygen stable isotope measurements in planktic foraminifera the investigated sites in the surroundings of the faroe islands are located in the transitional area where surface waters of subpolar and subtropical origin mix before entering the arctic mediterranean in these areas large amplitude millennial variability in the characteristics of the upper water column appears modulated by changes in the intensity of the subpolar gyre circulation from 7 8 to 6 ka bp faunal records indicate a deep mixed layer which in conjunction with lighter 18 o values suggest that the inflowing atlantic waters were dominated by a relatively cooler and fresher water mass reflecting a strengthening of the subpolar gyre under conditions of enhanced positive nao like forcing and reduced meltwater input a shift in the hydrographic conditions occurred during the mid holocene centered at 5 ka bp at this time increasing upper water column stratification and the incipient differentiation of the stable isotopic signal of the iceland faroe and faroe shetland surface water masses suggest increasing influx of warmer more saline surface waters from the subtropical gyre as subpolar gyre circulation weakened the mid holocene decline in subpolar gyre strength is presumably related to a shift toward a low state of the nao like forcing associated with decreased solar irradiance later in the holocene from 4 ka bp to present the increased frequency and reduced amplitude of the surface hydrographic changes reflect corresponding fluctuations in subpolar gyre circulation these high frequency oscillations in subpolar gyre strength suggest increased surface circulation sensitivity to moderate freshwater fluxes to the labrador irminger sea basin highlighting the importance of the salinity balance in modulating subpolar gyre dynamics particularly under conditions of low nao atmospheric forcing

subpolar mode water formation traced by neodymium isotopic composition 1 the formation of subpolar mode water spmw is documented by combining neodymium isotopic composition nd ic as a conservative tracer with salinity temperature and o2 concentration nd ic reveals a more pronounced eastward extension of the labrador current particularly its subarctic intermediate water component than the other parameters nd ic also suggests that waters flowing through the denmark strait do not significantly contribute to the spmw formation a two end member mixing involving subarctic intermediate water and maybe ultimately labrador current water and north atlantic central water is proposed to explain spmw formation these results provide important insight into the processes controlling the nd ic in the north atlantic subpolar gyre which could provide a means for reconstructing the northward extension of the gulf stream in the past

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

magma intrusion near volcan tancitaro evidence from seismic analysis abstract between may and june 2006 an earthquake swarm occurred near volcan tancitaro in mexico which was recorded by a temporary seismic deployment known as the mars network we located 1000 events from this seismic swarm previous earthquake swarms in the area were reported in the years 1997 1999 and 2000 we relocate and analyze the evolution and properties of the 2006 earthquake swarm employing a waveform cross correlation based phase repicking technique hypocenters from 911 events were located and divided into eighteen families having a correlation coefficient at or above 0 75 90 of the earthquakes provide at least sixteen phase picks we used the single event location code hypo71 and the p wave velocity model used by the jalisco seismic and accelerometer network to improve hypocenters based on the correlation adjusted phase arrival times we relocated 121 earthquakes which show clearly two clusters between 9 10 km and 3 4 km depth respectively the average location error estimates are