the control of polar haloclines by along isopycnal diffusion in climate models increasing the value of along isopycnal diffusivity in a coupled model is shown to lead to enhanced stability of north atlantic deep water nadw formation with respect to freshwater fw perturbations this is because the north atlantic na surface salinity budget is dominated by upward salt fluxes resulting from winter convection for low values of along isopycnal diffusivity whereas along isopycnal diffusion exerts a strong control on na surface salinity at higher diffusivity values shutdown of wintertime convection in response to a fw pulse allows the development of a halocline responsible for the suppression of deep sinking in contrast to convection isopycnal salt diffusion proves a more robust mechanism for preventing the formation of a halocline as surface freshening leads only to a flattening of isopycnals leaving at least some diffusive removal of anomalous surface fw in place as a result multiple equilibria are altogether absent for sufficiently high values of isopycnal diffusivity furthermore the surface salinity budget of the north pacific is also dominated by along isopycnal diffusion when diffusivity values are sufficiently high leading to a breakdown of the permanent halocline there and the associated onset of deep water formation

mechanisms controlling the sensitivity of the atlantic thermohaline circulation to the parameterization of eddy transports in ocean gcms abstract the authors identify and describe the important dynamical mechanisms that explain the significant sensitivity of the atlantic thermohaline circulation to the parameterization of heat and salt transports by mesoscale eddies in numerical models in particular the effects of the gent mcwilliams gm scheme which has a strong flattening effect on isopycnals and a simple horizontal diffusion scheme are considered and compared two control runs one with each scheme exhibit very different circulations and density structures to analyze the dynamical reasons for the differences between the control runs a number of numerical experiments with regionally varying diffusion coefficients are carried out emphasizing the effects of different schemes in key regions the main effect of eddies in the southern ocean in nature is to shoal the subsurface isopycnal surfaces thus increasing the density of the northward inflow of relatively dense intermediate waters into the atlantic as will be seen this is more

abrupt changes in antarctic intermediate water circulation over the past 25 000 years the formation and circulation of antarctic intermediate water has varied over glacial interglacial timescales a neodymium record from the atlantic ocean basin suggests that changes in circulation may have been driven by changes both in antarctic intermediate water formation in the southern ocean and in the strength of north atlantic meridional overturning

evidence for muon induced production of 10be in near surface rocks from the congo distributions of 10be in vein quartz from a lateritic weathering profile from the congo provide evidence for significant in situ production of 10be by reactions other than neutron induced spallation after consideration of geomorphological and nuclear processes which might contribute to the observed depth variability we conclude that cosmic ray muon induced reactions are the most likely mechanism and that such reactions lead to 1 to 3 of 10be production at the surface this small muon induced component has only minor implications for the use of 10be for quantification of exposure histories of surficial rocks nevertheless the muogenic component has the potential to expand the utility of cosmogenic nuclides in examination of surficial processes overcoming some of the limitations associated with the use of the neutron produced component

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

pushing the boundaries glacial interglacial variability of intermediate and deep waters in the southwest pacific over the last 350 000 years glacial interglacial changes in southern ocean s air sea gas exchange have been considered as important mechanisms contributing to the glacial interglacial variability in atmospheric co2 hence understanding past variability in southern ocean intermediate to deep water chemistry and circulation is fundamental to constrain the role of these processes on modulating glacial interglacial changes in the global carbon cycle our study focused on the glacial interglacial variability in the vertical extent of southwest pacific antarctic intermediate water aaiw we compared carbon and oxygen isotope records from epibenthic foraminifera of sediment cores bathed in modern aaiw and upper circumpolar deep water ucdw 943 2066 m water depth to monitor changes in water mass circulation spanning the past 350 000 years we propose that pronounced freshwater input by melting sea ice into the glacial aaiw significantly hampered the downward expansion of southwest pacific aaiw consistent with climate model results for the last glacial maximum this process led to a pronounced upward displacement of the aaiw ucdw interface during colder climate conditions and therefore to an expansion of the glacial carbon pool

relative geomagnetic intensity of the field during the last 140 ka relative magnetic field palaeointensities have been obtained from three marine cores in the somali basin western indian ocean the correlation between the records based on changes in the low field susceptibility and the inclination is extremely detailed a time versus depth correlation has been established from the 18o record the remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes coercivities and mineralogy the synthetic record of relative palaeointensity has been extended to cover the last 140 ka the quasi cyclic pattern observed during the last 80 ka confirms the results previously obtained in the mediterranean sea and thereby establishes its dominantly dipolar character between 140 and 80 ka the field intensity exhibits a much longer cycle spectral analysis of the intensity records shows the presence of two dominant periodicities but the length of the signal is too short to document the presence of a stationary process

origin of the oslo graben in relation to the hercynian alleghenian orogeny and lithospheric rifting in the north atlantic abstract the contemporaneous extrusion of basalts in the oslo graben and intrusion of dolentes in northern britain and southern sweden at 295 ma calls for a common explanation two hypotheses are investigated 1 1 the graben and dykes resulted from extensional stresses associated with progressive lithosphere separation to the northwest of europe in the late carboniferous 2 2 the dykes and therefore the graben were related in some way to oblique collision of plates in the hercynian orogeny which was developing to the south the first hypothesis is mechanically more satisfying and makes a number of testable predictions it states that in late carboniferous times the lithosphere separated in two places along the orogenic grain to the west of britain and norway the two embryonic oceanic rifts were divided by thick cold lithosphere with an archaean crust to the north of scotland extensional stresses were focussed in this region fanning out in an arcuate zone to the south and east causing failure where the lithosphere was relatively thin in norway the strain was restricted to a zone previously thinned and weakened in the early palaeozoic i e the oslo graben in britain the caledonoid grain is oblique to the expected direction of extensional strain and a dyke swarm trending e w and about 300 km wide was formed the first hypothesis predicts that the dykes should die out to the west but continue along an arc and widen to the east northeast under the western north sea interpretation of aeromagnetic maps shows that the dykes behave as predicted by hypothesis one but that their trends and extent are at variance with the expectations of hypothesis two the apparent contradiction of rifting to the northwest of europe occurring at the same time as compression and oblique collision in the heart of europe is resolved in principle by two plate tectonic reconstructions of pangaea drawn for late carboniferous and early permian times the oslo graben was conceived in mid carboniferous times and born towards the end of the carboniferous as the thin lithosphere failed and allowed the extrusion of basalt magma it was then abandoned as the two collinear oceanic rifts to the west and northwest joined up but the graben carried on a life of its own as the mafic magma cushion caught in the density trap beneath the crust continued to differentiate and alkali magma pods rose buoyantly to freeze in the upper crust or extrude on the graben floor

hydrographic biases in global coupled climate models and their relation to the meridional overturning circulation comparison of the volumetric s distribution of models participating in the climate model intercomparison project 3 cmip3 indicates that these models differ widely in their ability to represent the thermohaline properties of water masses relationships between features of the quasi equilibrium hydrographic mean state of these models and aspects of their overturning circulations are investigated this is achieved quantitatively with the help of seven diagnostic hydrographic stations these few stations were specifically selected to provide a minimalist schematic of the global water mass system relationships between hydrographic conditions in the north atlantic measured with a subset of these stations suggest that hydrographic properties in the subpolar north atlantic are set by the circulation field of each model pointing towards deficiencies in the models ability to resolve the gulf stream north atlantic current system as a major limitation since diapycnal mixing and viscosity parameterizations differ across cmip3 models and exert a strong control on the overturning it is likely that these architectural differences ultimately explain the main across model differences in overturning circulation temperature and salinity in the north atlantic the analysis of properties across the quasi equilibrium states of the cmip3 models agrees with previously reported relationships between meridional steric height gradients or horizontal density contrasts at depth and the strength of the deep water cell robust relationships are also found in the southern ocean linking measures of vertical stratification with the strength of the abyssal circulations across the cmip3 models consistent correlations between aspects of the quasi equilibrium hydrography in the southern ocean and the sensitivity of the abyssal cell to increasing radiative forcing by 2100 were found using these relations in conjunction with modern hydrographic observations to interpolate the fate of the abyssal cell suggests that the southern abyssal cell may decrease by roughly 20 by the end of the century similar systematic relationships between the quasi equilibrium hydrographic states of the models and the sensitivity of their atlantic deep water cell could not be found

subsurface exploration via satellite structure visible in seasat images of north sea atlantic continental margin and australia satellite observations of the sea surface height are sensitive to crustal structure overcoming the inability of previous remote sensing techniques to penetrate the earth s surface seasat gravity images have been compared with the structure of offshore sedimentary basins in northwestern europe seasat images display primary structural features e g the anglo dutch and smaller basins down to the scale of the manx furness basin and numerous secondary structural features e g the halibut horst additionally because of their synoptic viewpoint seasat images reveal features hard to distinguish i cite as an example a basement ridge extending from scotland to norway in position to have controlled marine circulation during deposition of the principal source rock wh ch is equivalent to the kimmeridge clay in north america and south america seasat images have been used to examine the relation of the atlantic fracture zones to the structure of the continental margin of the united states and brazil in australia known basins of the northwest continental margin and the bass strait have been compared with seasat projections of shallow structure in comparison with reconnaissance seismic magnetics and ship borne gravity surveys seasat images are valuable in discovering the presence of thick sediment accumulations seasat images are potentially more valuable than seismic and magnetics in the presence of basement masking by intrasectional volcanics or massive carbonates in known basins seasat data deliver a uniform synoptic view permitting the identification of large structural features in this form these images give the structural geologist a tool like that given the stratigrapher by regional seismic sections in remote unexplored regions seasat images provide a means of optimizing the layout of reflection seismic the gravity data from seasat offer the first of increasingly accurate structural images to be expected from altimetric satellites

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

regional distribution of volcanism within the north atlantic igneous province an overview of the distribution of volcanic facies units was compiled over the north atlantic region the new maps establish the pattern of volcanism associated with breakup and the initiation of seafloor spreading over the main part of the north atlantic igneous province naip the maps include new analysis of the faroe shetlands region that allows for a consistent volcanic facies map to be constructed over the entire eastern margin of the north atlantic for the first time a key result is that the various conjugate margin segments show a number of asymmetric patterns that are interpreted to result in part from pre existing crustal and lithospheric structures the compilation further shows that while the lateral extent of volcanism extends equally far to the south of the iceland hot spot as it does to the north the volume of material emplaced to the south is nearly double of that to the north this suggests that a possible southward deflection of the iceland mantle plume is a long lived phenomenon originating during or shortly after impact of the plume

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

a diagnostic indicator of the stability of the atlantic meridional overturning circulation in ccsm3 a diagnostic indicator dmov is proposed in this paper to monitor the stability of the atlantic meridional overturning circulation amoc the dmov is a diagnostic for a basinwide salt advection feedback and defined as the difference between the freshwater transport induced by the amoc across the southern border of the atlantic ocean and the overturning liquid freshwater transport from the arctic ocean to the north atlantic as validated in the community climate system model version 3 ccsm3 for an amoc in the conveyor state a positive dmov freshwater convergence in the atlantic basin indicates a monostable amoc and a negative dmov freshwater divergence indicates a bistable amoc based on dmov the authors investigate the amoc stability in the last glacial maximum lgm and analyze the modulation of the amoc stability by an open closed bering strait moreover the authors estimate that the real amoc is likely to be bistable in the present day since some observations suggest a negative dmov freshwater divergence is currently in the atlantic basin however this estimation is very sensitive to the choice of the observational data

instability of glacial climate in a model of the ocean atmosphere cryosphere system in contrast to the relatively stable climate of the past 10 000 years during glacial times the north atlantic region experienced large amplitude transitions between cold stadial and warm interstadial states in this modeling study we demonstrate that hydrological interactions between the atlantic thermohaline circulation thc and adjacent continental ice sheets can trigger abrupt warming events and also limit the lifetime of the interstadial circulation mode these interactions have the potential to destabilize the thc which is already more sensitive for glacial conditions than for the present day climate thus providing an explanation for the increased variability of glacial climate

age and structure of the southern rockall trough new evidence abstract the rockall trough separates the rockall plateau microcontinent from the shelf and slope west of the british isles the structure and age of the trough has been the source of considerable discussion although widely considered to be of oceanic origin postulated ages for the spreading range from permian to cretaceous new seismic profiles linked to the ipod sites in the bay of biscay and to oceanic anomalies of known age are used to present a new assessment of the age and structure of the southern rockall trough it is concluded that about 120 km of ocean crust is present in the trough and that spreading took place in the albian maastrichtian interval

lead isotope evidence for the structure of the rockall dipping reflector passive margin the passive continental margin south west of rockall plateau is characterized by a thick sequence of oceanward dipping seismic reflectors during leg 81 of the deep sea drilling project these reflectors were sampled at site 553 and proved to consist almost exclusively of basalt here we present lead isotope data which indicate that these basalts may have been contaminated by ancient uranium depleted continental crust or alternatively derived from a sub continental lithospheric mantle source in either case the implications are that the basalts of the south west rockall plateau formed by eruption through and onto continental basement not by subaerial seafloor spreading this conclusion is in accord with gravity models of the area which predict stretched continental crust beneath the dipping reflector sequence

a review of the australian new zealand sector of the southern ocean over the last 30ka aus intimate project the australia new zealand region of the southern ocean is influenced by several of the major global water masses of the oceans and is the prime entry point for cold deep waters into the pacific basin during the last glacial there was increased sea ice extent around antarctica as far north as 55 s as well as increased iceberg presence inferred from ice rafted debris evidence from microfossil assemblages suggests that sea surface temperatures sst were up to 7 c cooler consistent with recent estimates of cooling for new zealand derived from glacier modelling and other terrestrial proxies the subtropical front stf subantarctic front saf and polar front pf had migrated north except where the position of the fronts were controlled bathymetrically despite the potential for iron fertilisation by increased dust input into the ocean during the glacial there is limited evidence for higher total biological productivity in the pacific sector of the southern ocean the altered oceanic circulation during the glacial also decreased nutrients in the surface waters and affected the outgassing of co 2 this contributed to an increased storage of co 2 in the deep waters and lowering of the carbonate lysocline during the deglaciation sea ice retreat and sst increased rapidly at 18 ka roughly synchronous with the reinvigoration of deep water circulation in the southern ocean and the release of co 2 stored in the deep waters the gradient in carbon isotopes 13 o benthic between antarctic intermediate water aaiw and lower circumpolar deep water lcdw was greatest at the start of the deglaciation suggesting that the aaiw ventilation preceded lcdw ventilation or there was a significant change in air sea fractionation of 13 o benthic there was a slight enrichment in 18 o planktic decrease in ssts and a reduction in intermediate and deep water circulation between 14 and 12 5 ka bp during the antarctic cold reversal acr coincident with glacier advances in the new zealand southern alps and other terrestrial records of cooling following the acr there was a second more minor release of deep water co 2 most likely related to reinvigoration of north atlantic deep water nadw from 10 ka onwards the modern intermediate and deep water circulation was established air temperatures and ssts reached a maximum in the early holocene with the stf located at its most southerly position and there was a widespread retreat of antarctic ice sheets to their modern position a decline to modern sst and air temperatures in the mid to late holocene followed while millennial cycles overprinted on the sst and 18 o planktic records may be the result of subtle changes in the position and strength of the westerly winds during the holocene

the development of volcanic sequences at rifted margins new insights from the structure and morphology of the voring escarpment mid norwegian margin on the voring margin offshore mid norway paleogene continental breakup was characterized by the extrusion of large volumes of flood basalts erupted in different depositional environments the transition from subaerial to submarine emplacement environment is marked by the formation of the voring escarpment which records the early encroachment of flood basalt into the basin and the buildup of a lava delta system the increased availability of new and reprocessed high quality seismic data allows a more detailed characterization of the along strike and across strike continuity and variability of the different volcanic seismic facies units detailed seismic interpretation shows that the 350 km long ne sw trending voring escarpment is a prominent feature along the voring margin with a height ranging between 200 and 1600 m structurally the voring escarpment is segmented along strike into five segments e1 e5 with different controlling factors leading to variation in accommodation space relative sea level change and magma supply are the major controlling factors for segments e2 and e4 which are characterized by a well developed lava delta system and significant escarpment height tectonic movements along the jan mayen fracture zone resulted in second order segmentation of the e1 segment into pseudoescarpments with a very thin lava delta system and limited escarpment height segments e3 and e5 situated along the flanks of cretaceous paleocene highs are controlled by the structural highs which were possibly reactivated during breakup time our mapping results provide crucial information about the paleogeography and yield important information regarding the paleo water depth and depocenter locations prior to and during the breakup of the voring margin

in situ10be 26al exposure ages at meteor crater arizona a new method of dating the surface exposure of rocks from in situ production of 10be and 26al has been applied to determine the age of meteor crater arizona a lower bound on the crater age of 49 200 1 700 years has been obtained by this method

use of marine controlled source electromagnetic sounding for sub basalt exploration the problem of imaging sedimentary structure obscured by high velocity layers such as carbonate basalt or salt using conventional seismic techniques is well known when this problem is encountered in offshore areas marine electromagnetic data can provide valuable alternative and complementary constraints on the structure we concentrate on the use of marine controlled source electromagnetic csem sounding in the frequency domain the feasibility of mapping sub basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice as a starting point the problem is addressed in terms of 1d resistivity structure we show that sub basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown the presence of small scale inhomogeneities in the basalt which cause much of the scattering in seismic data is shown to have no appreciable effect on the ability of the csem data to detect the sediments the modelling is then extended to 2 5 dimensions again the presence of sub basalt sediments can be readily detected and their properties and geometry quantified especially if the electromagnetic data are combined with constraints from complementary geophysical techniques

meltwater pulse 1a from antarctica as a trigger of the bolling allerod warm interval meltwater pulse 1a mwp 1a was a prominent feature of the last deglaciation which led to a sea level rise of 20 meters in less than 500 years concurrent with mwp 1a was the onset of the bolling allerod interstadial event 14 600 years before the present which marked the termination of the last glacial period previous studies have been unable to reconcile a warm northern hemisphere with mwp 1a originating from the laurentide or fennoscandian ice sheets with the use of a climate model of intermediate complexity we demonstrate that with mwp 1a originating from the antarctic ice sheet consistent with recent sea level fingerprinting inferences the strength of north atlantic deep water nadw formation increases thereby warming the north atlantic region and providing an explanation for the onset of the bolling allerod warm interval the established mode of active nadw formation is then able to respond to subsequent freshwater forcing from the laurentide and fennoscandian ice sheets setting the stage for the younger dryas cold period the melting of continental ice sheets during the last deglaciation provided a freshwater source to the ocean that affected global sea level and the strength of the thermohaline circulation an exceptionally large melting event inferred from far field relative sea level records occurred

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

tracer dating and ocean ventilation the interpretation of transient tracer observations depends on difcult to obtain information on the evolution in time of the tracer boundary conditions and interior distributions recent studies have attempted to circumvent this problem by making use of a derived quantity age based on the simultaneous distribution of two complementary tracers such as tritium and its daughter helium 3 the age is defined with reference to the surface such that the boundary condition takes on a constant value of zero we use a two dimensional model to explore the circumstances under which such a combination of conservation equations for two complementary tracers can lead to a cancellation of the time derivative terms an interesting aspect of this approach is that mixing can serve as a source or sink of tracer based age we define an idealized ventilation age tracer that is conservative with respect to mixing and we explore how its behavior compares with that of the tracer based ages over a range of advective and diffusive parameters transient tracers such as tritium and tritiugenic helium 3 chlorofiuorocarbons especially cfc 11 and cfc 12 and radiocarbon produced by nuclear bomb tests have entered the oceans primarily during the past 30 years the inherent time scale of these tracers matches very well typical time scales for thermocline ventilation as well as other important oceanic processes such as deep water formation they have been used with some success to study the time scales of thermocline ventilation e g sarrniento 1983 jenkins 1987 and oxygen utilization rates our e g jenkins 1977 1980 1982 1987 as well as other processes such as equatorial upwelling rates broecker et al 1978 quay et al 1983 wunsch 1984 and transport of upper north atlantic deep water weiss et al 1985 the problem of coping with the nonsteady state aspect of these tracers has been dealt with in many ways few of them truly satisfactory for example rooth and ostlund 1972 chose to estimate the diapycnic diffusivity in a region where there was some evidence that the tracer distributions were approximately in steady state at the time of their measurements wunsch 1984 used a similar steady state assumption in his study of equatorial upwelling using bomb radiocarbon such an argu ment cannot be applied in most regions of the ocean however indeed one can question whether it applies even in the above situations therefore one generally would expect that it would be necessary to know something about the evolution of the tracer concentration in time as well as the time history of the boundary conditions for that tracer over the same span of time for example sarrniento s 1983 tritium box model of the thermocline solved the conserva tion equation for the period of time between 1952 before nuclear bomb tests had injected a significant amount of tritium into the oceans and 1972 when a single realization of the tracer field was obtained by a series of field studies the surface concentration time history of dreisigacker and roether 1978 was used as a boundary condition such an l now at deutsche forschungs und versuchasanstalt ftir luft

basin structure on the norwegian margin from analysis of digitally recorded sonobuoys during r v robert d conrad cruise 21 a large number of sonobuoys were recorded in the voring more and adjacent basins at the margin off norway the digital sonobuoy data have been transformed into the domain of intercept time tau and ray parameter p and inverted by the tau sum method to obtain better definition of the near seafloor velocities using also the precritical energy a move out correction has been applied to the tau p data conventional analyses of the x t data were also performed in general the various types of analyses complement each other and should be used interactively together with forward modeling to obtain the optimal velocity depth function the sonobuoy results have outlined in a regional sense the pre cenozoic sedimentary basin at the margin this basin attains a maximum sediment thickness of 8 10 km of which only the upper half is defined by seismic reflection profiling at the outer margin the marginal escarpments form major discontinuities in the lateral velocity distribution the survey has proven that sonobuoy profiles recorded during standard seismic reflection profiling in major marginal basins may provide important new information particularly about the deeper parts of the basins that is not directly available from the reflection profiles

seasonal flow reversals of intermediate waters in the canary current system east of the canary islands two 9 yr current meter and salinity records together with climatological data reveal marked seasonal variability of the intermediate flow at a key location in the canary basin the region is characterized by a summer july september and early fall october northward progression of antarctic intermediate water followed by a fall november december intense flow reversal of mediterranean water a sverdrup type model confirms that these movements are associated with stretching or shrinking of the intermediate water strata

mechanisms forcing an antarctic dipole in simulated sea ice and surface ocean conditions abstract the mechanisms forcing variability in southern ocean sea ice and sea surface temperature from 600 years of a control climate coupled model integration are discussed as in the observations the leading mode of simulated variability exhibits a dipole pattern with positive anomalies in the pacific sector associated with negative anomalies in the atlantic it is found that in the pacific ocean circulation changes associated with variable wind forcing modify the ocean heat flux convergence and sea ice transport resulting in sea surface temperature and sea ice anomalies the pacific ice and ocean anomalies persist over a number of years due to reductions in ocean shortwave absorption reinforcing the initial anomalies in the atlantic sector no single process dominates in forcing the anomalies instead there are contributions from changing ocean and sea ice circulation and surface heat fluxes while the absorbed solar radiation in the atlantic is modified by the changing surface albedo the anomalies

variability in north pacific intermediate and deep water ventilation during heinrich events in two coupled climate models the responses of north pacific intermediate and deep water ventilation and ocean biogeochemical properties to northern north atlantic glacial freshwater perturbations are evaluated with a coupled atmosphere ocean general circulation model miroc and an earth system model of intermediate complexity loveclim when the atlantic meridional overturning circulation amoc is weakened as a result of the north atlantic freshwater discharge both models simulate subthermocline and intermediate water warming in the pacific ocean the sensitivities of the pacific meridional overturning circulation pmoc to amoc weakening differ significantly between the two models miroc simulates a small enhancement of the deep sinking branch of the pmoc in the north pacific on the contrary the loveclim freshwater experiment exhibits intensified deep water formation in the north pacific associated with a maximum transport change of 19 sv despite the significant differences in ocean circulation response both models successfully reproduce high oxygen and low nutrient conditions of intermediate and deep waters in accordance with sediment core based paleoproxy reconstructions from the north pacific and bering sea during heinrich event 1 emergence of younger intermediate and deep water in the north pacific can be partly attributed to an overall enhanced mixing as well as intensified overturning circulation of the subpolar north pacific our models simulate broad features observed in several paleoproxy data of the pacific ocean biological production decrease in northern japan cooling in the western north pacific ocean and the southward shift of the pacific intertropical convergence zone

antarctic sea ice and the control of pleistocene climate instability a hypothesis is presented for the origin of pleistocene climate instability based on expansion of antarctic sea ice and associated changes in the oceans salinity structure the hypothesis assumes that thermohaline overturning is dominated by the reconfigured conveyor of toggweiler and samuels 1993b in which deepwater upwelling is restricted to high southern latitudes the reconfigured conveyor is shown to be potentially stabilized in an on mode by precipitation at high southern latitudes and potentially destabilized into on and off modes by the counteracting influence of antarctic sea ice the mechanism is clarified by the use of a hydraulic analogue we hypothesize that this mechanism accounts for dominant patterns of thermohaline overturning and climate instability between pleistocene warm and cold periods the hypothesis is shown to be consistent with a range of paleoceanographic evidence and to potentially account for details of observed rapid climate changes during glacial and interglacial periods including aspects of interhemispheric timing

stochastic models of the meridional overturning circulation time scales and patterns of variability the global meridional overturning circulation moc varies over a wide range of space and time scales in response to fluctuating weather perturbations that may be modelled as stochastic forcing th

enhanced arabian sea intermediate water flow during glacial north atlantic cold phases abstract during the last glacial period polar ice cores indicate climate asynchrony between the poles at the millennial time scale yet surface ocean circulation in large parts of the globe varied in tune with greenland temperature fluctuations suggesting that any anti phase behavior to a substantial degree must lie in the deeper global ocean circulation which is poorly understood outside the atlantic ocean here we present data from the north western indian ocean which indicate that the timing of maxima in northward extensions of glacial antarctic intermediate water gaaiw coincides with dramatically reduced thermohaline overturn in the north atlantic associated with the heinrich ice surge events he the repeated expansion of the gaaiw during hes recorded far north of the equator in the arabian sea suggests that southern hemisphere driven intermediate water mass variability forms an integral part of the inter hemisphere asynchronous climate change behavior at the millennial time scale

early opening history of the north atlantic i structure and origin of the faeroe shetland escarpment summary marine geophysical surveys show that the escarpment is the buried feather edge of a thick pile of flood basalts of early eocene age overlying a thinner more widespread layer of basalts of late palaeocene age the escarpment does not therefore define the continent ocean boundary in the southern norwegian sea but instead marks the contemporary shoreline separating terrestrially erupted basalt flows in the north from a restricted shallow water shelf to the south the basalts overlie 5 6 km of mesozoic sediments which have completely buried a large conical flat topped seamount of similar dimensions to anton dohrn and rosemary bank we call this newly postulated body the brendan seamount the mesozoic sediments are at least as old as early cretaceous in age therefore precluding the possibility that the mid cretaceous seafloor spreading episode supposed by some to have created the rockall trough could have also created the thin crust inferred to underlie the faeroe shetland trough and mbre basin

an idealized model of the world ocean part i the global scale water masses abstract a primitive equation three dimensional numerical model of the ocean employing idealized versions of the real topography and surface boundary conditions is used to study the water mass structure of the world ocean in particular the response of the model to three fundamental changes in boundary conditions is investigated in an attempt to identify the mechanisms in the model which are responsible for the establishment of the largest scale features of the global water mass structure with the drake passage closed thermohaline driving alone and a fresh north atlantic surface salinity specified only the coarsest aspects of the observed t and s structure are reproduced and the entire world ocean below the thermocline is dominated by water formed at the southern boundary the salinity configuration in particular lacks much of its observed structure in this case when the drake passage is opened the resulting circumpolar flow serves to isolate the extreme southern ocean this allows waters of no

the release of 14 c depleted carbon from the deep ocean during the last deglaciation evidence from the arabian sea article i nfo during the last deglaciation the concentration of co2 in the atmosphere increased and the radiocarbon activity 14 c of the atmosphere declined in two steps corresponding in timing to heinrich stadial 1 and the younger dryas these changes have been attributed to the redistribution of 14 c depleted carbon from the deep ocean into the upper ocean and atmosphere recently reconstructions of 14 c in intermediate waters of the eastern tropical pacific have revealed pulses of very old water during the deglaciation consistent with the release of 14 c depleted carbon from the deep ocean at this time here we present reconstructions of intermediate water 14 c from the northern arabian sea near the coast of oman these reconstructions record significant aging of intermediate waters in the arabian sea during heinrich stadial 1 and to a lesser extent during the younger dryas the timing and magnitude of 14 c depletion in the arabian sea during heinrich stadial 1 is very similar to that previously observed in the eastern north pacific near baja california indicating that similar mechanisms were involved in controlling 14 c at these two sites the most parsimonious explanation of the 14 c records from the arabian sea and baja california remains the release of 14 c depleted carbon from the deep ocean by renewal of upwelling and mixing in the southern ocean these

south atlantic intermediate water advances into the north east atlantic with reduced atlantic meridional overturning circulation during the last glacial period the nd isotopic composition end of seawater and cold water coral cwc samples from the gulf of cadiz and the alboran sea at a depth of 280 827 m were investigated in order to constrain middepth water mass dynamics within the gulf of cadiz over the past 40 ka end of glacial and holocene cwc from the alboran sea and the northern gulf of cadiz reveals relatively constant values 8 6 to 9 0 and 9 5 to 10 4 respectively such values are similar to those of the surrounding present day middepth waters from the mediterranean outflow water mow end 9 4 and mediterranean sea water msw end 9 9 in contrast glacial end values for cwc collected at thermocline depth 550 827 m in the southern gulf of cadiz display a higher average value 8 9 0 4 compared to the present day value 11 7 0 3 this implies a higher relative contribution of water masses of mediterranean msw or south atlantic origin east antarctic intermediate water eaaiw our study has produced the first evidence of significant radiogenic end values 8 at 19 23 24 and 27 ka which are coeval with increasing iceberg discharges and a weakening of atlantic meridional overturning circulation amoc since mow end values remained stable during the last glacial period it is suggested that these radiogenic end values most likely reflect an enhanced northward propagation of glacial eaaiw into the eastern atlantic basin

on the link between the two modes of the ocean thermohaline circulation and the formation of global scale water masses a close link between the formation of global scale water masses such as north atlantic deep water nadw and antarctic intermediate water aaiw and two stable modes of the thermohaline circulation thc is investigated in a coupled model in the upper 2 3 km of the atlantic the thc modes are characterized by meridional overturning circulations of opposite sign with either a dominance of the aaiw cell over the nadw cell off thc mode or vice versa on thc mode a transition between these thc modes is controlled by the relationship between the densities in the source regions of formation of aaiw and nadw water masses this is shown by applying a freshwater perturbation in the region of enhanced aaiw formation in the southern ocean to obtain a hysteresis loop of the nadw circulation transitions between the two modes of the thc occur when the densities in the source regions of aaiw and nadw become comparable to each other

cosmogenic ages for earthquake recurrence intervals and debris flow fan deposition owens valley california model exposure ages beryllium 10 aluminum 26 of boulders on an offset debris flow fan yield an earthquake recurrence interval between 5800 and 8000 10be 26al years for a strand of the owens valley fault in california which last ruptured in an earthquake of moment magnitude 7 5 in 1872 cosmogenic age estimates for this and several nearby fan surfaces flanking the eastern sierra nevada are consistent with stratigraphic relations and suggest that these surfaces were abandoned after 1000 8000 and 21 000 10be 26al years ago the wide scatter and nonconcordance of 10be 26al ages on an older fan surface suggest that boulder erosion and lowering of the fan surface there have influenced apparent exposure ages

changes in the advection of antarctic intermediate water to the northern chilean coast during the last 970 kyr 1 the antarctic intermediate water aaiw is a key player in global scale oceanic overturning processes and an important conduit for heat fresh water and carbon transport the aaiw past variability is poorly understood mainly due to the lack of sedimentary archives at intermediate water depths we present records of benthic stable isotopes from sediments retrieved with the seafloor drill rig marum mebo at 956 m water depth off northern chile geob15016 27 29 48 s 71 07 58 w that extend back to 970 ka the sediments at this site are presently deposited at the boundary between aaiw and pacific deep water pdw for previous peak interglacials our results reveal similar benthic 13c values at site geob15016 and of a newly generated stack of benthic 13c from various deep pacific cores representing the average pdw this suggests unlike today the absence of aaiw at the site and the presence of nearly pure pdw in contrast more positive 13c values at site geob15016 compared to the stack imply a considerable aaiw contribution during cold phases of interglacials and especially during glacials besides we used three short sediment cores to reconstruct benthic 13c values from the aaiw core during the last glacial and found a 13c signature similar to today s assuming that this was the case also for the past 970 kyr we demonstrate that sea level changes and latitudinal migrations of the aaiw formation site can only account for about 50 of the full range of past 13c increases at site geob15016 during cold periods other processes that could explain the remaining of the positive 13c anomalies are increases in glacial aaiw production and or deeper convection of the aaiw with respect to preceding interglacials

cretaceous tectonics off mid norway implications for the rockall and faeroe shetland troughs between 62 and 68 n on the norwegian continental margin the hypothesis of a cretaceous or permian ocean extending northwards from the rockall trough can be tested with data from commercial reflection seismic profiles two major mesozoic basin systems occur the helgeland basin is a late triassic and early jurassic depocentre with a thin upper jurassic and cretaceous cover except in two late cretaceous sub basins the more lnner voring basin system is a middle to late cretaceous depocentre in which the pre cretaceous section is deeply buried and rarely seen on seismic profiles the two basin systems are separated by a complex zone of faulted 9marginal highs9 active in jurassic times and reactivated during cretaceous times pre cenomanian lavas or sills are interpreted within the more basin and a pre cretaceous block faulted sequence can be identified across the inner voring basin except in a narrow axial zone these features suggest the presence of an extremely narrow zone of oceanic crust of cretaceous age fault patterns confirm a major tectonic episode of mid cretaceous age the geology off mid norway supports a mid cretaceous age for sea floor spreading in the rockall trough but suggests that the generated oceanic crust narrowed northwards and that plate motion was partially accommodated by dextral transcurrent faulting reconstruction of jurassic tectonic trends in nw europe must allow for subsequent cretaceous distortion

the atlantic pacific seesaw abstract a global oceanic teleconnection of salinity meridional overturning circulation moc and climate of the north atlantic and north pacific is proposed simulations with a global climate model show that an extraction of freshwater from the pacific results not only in an increase of salinity there but also in a decrease of salinity in the atlantic as a result a pacific moc develops while the atlantic moc collapses without freshwater perturbation in the atlantic similarly an input of freshwater to the atlantic leads not only to a decrease of salinity there but also to an increase of salinity in the pacific the atlantic moc collapses whereas the pacific moc develops without freshwater perturbation in the pacific the mechanism behind this antiphase atlantic pacific relationship is the positive feedback between ocean circulation and salinity contrasts originally proposed by stommel to operate between low and high latitudes here the authors show that the same mechanism operates on the atlan

role of the drake passage in controlling the stability of the ocean s thermohaline circulation the role of a southern ocean gateway in permitting multiple equilibria of the global ocean thermohaline circulation is examined in particular necessary conditions for the existence of multiple equilibria are studied with a coupled climate model wherein stable solutions are obtained for a range of bathymetries with varying drake passage dp depths no transitions to a northern hemisphere nh overturning state are found when the drake passage sill is shallower than a critical depth 1100 m in the model described herein this preference for southern hemisphere sinking is a result of the particularly cold conditions of the antarctic bottom water aabw formation regions compared to the nh deep water formation zones in a shallow or closed dp configuration this forces an exclusive production of deep bottom water in the southern hemisphere increasing the depth of the drake passage sill causes a gradual vertical decoupling in atlantic circulation removing the influence of aabw from the upper 2000 m of the atlantic ocean when the dp is sufficiently deep this shifts the interaction between a north atlantic deep water nadw cell and an aabw cell to an interaction between an shallower antarctic intermediate water cell and an nadw cell this latter situation allows transitions to a northern hemisphere overturning state

precise cosmogenic 10be measurements in western north america support for a global younger dryas cooling event the inner titcomb lakes moraine in the wind river mountains wyoming has been dated with a precision of 5 using accelerator mass spectrometric measurements of cosmogenic 10be in surface samples from nine boulders resting on its crest the age of this moraine is 11 4 0 5 to 13 8 0 6 ka the range in age is due to the uncertainty in the 10be production rate and uncertainties in rock erosion rate and shielding of cosmic rays by snow additional exposure ages on bedrock and boulder surfaces that are stratigraphically younger and older than the inner titcomb lakes moraine conform with this age the exposure ages of the inner titcomb lakes moraine overlap the north atlantic younger dryas event 12 8 0 2 to 11 5 0 3 ka

dating soils and alluvium with cosmogenic 21ne depth profiles case studies from the pajarito plateau new mexico usa abstract 21 ne depth profiles measured from amalgamated samples of sand sized quartz from the quaternary bandelier tuff were used to estimate inheritance corrected exposure ages in stream terraces and an alluvial fan on the pajarito plateau stable alluvium and associated soils display a downward decreasing type i nuclide profile pattern bioturbated colluvium with uniform type iii nuclide concentrations has buried these alluvial surfaces another possible profile type with downward increasing nuclide concentrations type ii is described but was not found on the pajarito plateau we present mathematical models for each of the profile patterns relating nuclide concentration to inheritance time averaged bulk density and either exposure time or burial rate curve fits of type i profiles are used to estimate the age of alluvial landforms a burial correction technique for deposits buried by younger colluvium is also presented inheritance and burial corrected 21 ne surface exposure ages of 144 17 ka and 63 8 ka for two pajarito plateau stream terraces agree within error with independent soil profile development index dates of 160 ka and 68 78 ka a soil overlying the 50 60 ka el cajete pumice yields an inheritance corrected 21 ne exposure age of 44 11 ka that also agrees closely with a 14 c age on soil charcoal of 41 ka these results demonstrate that reliable cosmogenic 21 ne exposure ages can be obtained for fine grained alluvial surfaces with the depth profile technique

the role of the atlantic freshwater balance in the hysteresis of the meridional overturning circulation we have studied the response of the atlantic meridional overturning circulation to surface freshwater forcing using an ocean gcm coupled to an energy moisture balance atmosphere model the overturning collapses rapidly when a slowly increasing forcing applied to the north atlantic passes a positive threshold and spins up equally quickly when the forcing falls below a negative threshold this well known behaviour is referred to as hysteresis because the thresholds in forcing are different for the transitions in opposite directions however we argue that the behaviour of the atlantic salinity is more fundamental than the forcing hysteresis as a function of freshwater forcing occurs because the states with north atlantic overturning on and off each tend to reinforce their associated salinity distributions and inhibit the transition to the other state during the collapse the atlantic becomes less saline because of the import of 80 sv year of freshwater by ocean transports across 30 s during the spin up this freshwater is exported again we show that qualitatively similar hysteresis behaviour can be produced by perturbing the system without any net freshwater forcing the salinity flip flop is associated with the appearance and disappearance of a shallow reverse overturning circulation south of the equator which is present while the northern overturning is absent and may provide the mechanism for the ocean freshwater influx during collapse

palaeogene peraluminous magmatism crustal melting and continental breakup the erlend complex faeroe shetland basin ne atlantic well 209 3 1 located close to the centre of the tertiary erlend complex north of the shetland isles contains an unusual volcanic sequence in which thick basalts of n to t type morb affinity overlie highly peraluminous cordierite bearing dacites the dacites from the erlend complex are almost identical in terms of their petrography and major element trace element and isotope geochemistry to those of dacites from two previously described localities in the north atlantic and indicate the formation of the dacites by anatexis of aluminous upper crustal sediments which were probably pelitic in character the discovery of graphite in the erlend dacites suggests that carbonaceous shales may be implicated as parent material the geological settings of the erlend volcanic sequence and the other examples in the rockall trough and on the voring plateau correspond to intra continental pre break up rift systems underlain by thinned continental crust occurring close to the continental margin a possible petrogenesis for the dacites involves the partial melting of upper crustal sediments as a result of the extensive contemporaneous basic magmatism documented to have occurred within a wide area of the north atlantic continental margins during the initial stages of continental break up this mechanism suggests that the observed volcanic sequence may be more common than is obvious from the present very limited drilling evidence

relative and absolute dating of land surfaces abstract the dating of land surfaces has long posed problems for geomorphologists relative methods stratigraphic geomorphic topographic are sound and convincing exhumed forms may complicate identification and relationships for both epigene and etch forms have been buried and exhumed but in tectonically undisturbed areas the higher surfaces are older than those preserved at lower levels also surfaces have an age range the relationship of surfaces with volcanic deposits old shorelines and genetically related sedimentary sequences provides sound ages and correlation with dated duricrusts and faults is also useful there are no temporal limits to relative dating for the methods are equally applicable to the dating say of proterozoic surfaces as of those of pleistocene age the disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed the so called absolute physical numerical methods and especially those based on exposure age dating with in situ cosmogenic radionuclides are appealing because they produce direct numerical ages and appear to be widely applicable but there are severe temporal limitations and sampling problems complicate and may invalidate interpretation absolute age determinations must be consistent with the stratigraphic and geomorphologic settings the best results are obtained when physicists and earth scientists pool their knowledge and experience a background in local and regional geology is especially important

freshwater teleconnections and ocean thermohaline circulation 1 asymmetry of the atlantic and pacific sea surface salinity sss is recognized as an important element of the global ocean thermohaline circulation however a threshold of such asymmetry that may trigger a true global deep ocean conveyor has not yet been examined a combined effect of the atlantic pacific and the southern ocean surface salinity asymmetries also has not yet been clearly shown we address these issues and conclude that atlantic pacific sss asymmetry is one of the most critical elements for maintaining the global ocean conveyor our experiments suggest albeit preliminary that high latitudinal freshwater impacts as a mechanism of altering global ocean thermohaline circulation may be less effective than inter basin freshwater communications

changes in intermediate water conditions in the northern south china sea using globorotalia inflata over the last 20 ka intermediate circulation is one of the crucial controls of thermohaline changes in seawater due to the lack of data for some of the more significant indicators there has been little research into past intermediate waters in the south china sea scs in particular little is known of the thermohaline properties of intermediate water here we present paired mg ca and 18o data for the planktonic foraminifera globorotalia inflata from core ghe27l in the northern scs these data provide records of temperature and residual 18oseawater 18oresidual being used as a proxy for salinity to tentatively investigate changes in the intermediate water of the scs over the last 20 ka our results show that g inflata in the scs grew principally within the 200 450 m water depth interval thermohaline changes recorded by g inflata indicate warm and relatively freshening water during the last glacial maximum lgm 20 17 7 ka bp changing to warm and saline waters between 17 7 and 13 6 ka bp in general waters during the 13 6 8 3 ka bp period were characterized by high temperatures and salinity with obvious millennial scale fluctuations after which during the holocene they became cold and fresh we suggest that the temperature and salinity of the intermediate water of the scs recorded by g inflata in core ghe27l were possibly associated with a switch in intermediate water masses during the lgm the intermediate water in the northern scs was mainly impacted by north pacific intermediate water npiw but that subsequently the influence of antarctic intermediate water strengthened during the last deglaciation and pre boreal period 17 7 8 3 ka bp after this period npiw was the principal control of the intermediate water of the scs again

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

north atlantic deep water collapse triggered by a southern ocean meltwater pulse in a glacial climate state 1 it is generally accepted that surface freshwater anomalies in the southern ocean drive increases in north atlantic deep water nadw formation via a bipolar density see saw we find that a southern ocean freshwater pulse of comparable magnitude to meltwater pulse 1a shuts down instead of strengthens nadw in a glacial climate simulation unlike the modern day simulation the glacial experiment is associated with a more fragile north atlantic thermohaline circulation whereby freshwater anomalies that propagate into the north atlantic are able to dominate the bipolar density see saw meltwater pulses over the north atlantic and subsequent nadw shut down are often invoked to explain cold heinrich events appearing in the paleoclimate record our results suggest that triggers for nadw collapse may also originate from the southern hemisphere in glacial epochs once nadw collapses north pacific deep water develops consistent with a north pacific north atlantic see saw triggered from the southern ocean

sea level fingerprinting as a direct test for the source of global meltwater pulse ia the ice reservoir that served as the source for the meltwater pulse ia remains enigmatic and controversial we show that each of the melting scenarios that have been proposed for the event produces a distinct variation or fingerprint in the global distribution of meltwater we compare sea level fingerprints associated with various melting scenarios to existing sea level records from barbados and the sunda shelf and conclude that the southern laurentide ice sheet could not have been the sole source of the meltwater pulse whereas a substantial contribution from the antarctic ice sheet is consistent with these records

abrupt pre bolling allerod warming and circulation changes in the deep ocean several large and rapid changes in atmospheric temperature and the partial pressure of carbon dioxide in the atmosphere probably linked to changes in deep ocean circulation occurred during the last deglaciation the abrupt temperature rise in the northern hemisphere and the restart of the atlantic meridional overturning circulation at the start of the bolling allerod interstadial 14 700 years ago are among the most dramatic deglacial events but their underlying physical causes are not known here we show that the release of heat from warm waters in the deep north atlantic ocean probably triggered the bolling allerod warming and reinvigoration of the atlantic meridional overturning circulation our results are based on coupled radiocarbon and uranium series dates along with clumped isotope temperature estimates from water column profiles of fossil deep sea corals in a limited area of the western north atlantic we find that during heinrich stadial 1 the cool period immediately before the bolling allerod interstadial the deep ocean was about three degrees celsius warmer than shallower waters above this reversal of the ocean s usual thermal stratification pre dates the bolling allerod warming and must have been associated with increased salinity at depth to preserve the static stability of the water column the depleted radiocarbon content of the warm and salty water mass implies a long term disconnect from rapid surface exchanges and although uncertainties remain is most consistent with a southern ocean source the heinrich stadial 1 ocean profile is distinct from the modern water column that for the last glacial maximum and that for the younger dryas suggesting that the patterns we observe are a unique feature of the deglacial climate system our observations indicate that the deep ocean influenced dramatic northern hemisphere warming by storing heat at depth that preconditioned the system for a subsequent abrupt overturning event during the bolling allerod interstadial

carbon isotope evidence for changes in antarctic intermediate water circulation and ocean ventilation in the southwest pacific during the last deglaciation 1 deep sea sediment core fr1 97 gc 12 is located 990 mbsl in the northern tasman sea southwest pacific where antarctic intermediate water aaiw presently impinges the continental slope of the southern great barrier reef analysis of carbon 13c and oxygen 18o isotope ratios on a suite of planktonic and benthic foraminifera reveals rapid changes in surface and intermediate water circulation over the last 30 kyr during the last glacial maximum there was a large 13c offset 1 1 between the surface dwelling planktonic foraminifera and benthic species living within the aaiw in contrast during the last deglaciation termination 1 the 13cplanktonic benthic offset reduced to 0 4 prior to an intermediate offset 0 7 during the holocene we suggest that variations in the dominance and direction of aaiw circulation in the tasman sea and increased oceanic ventilation can account for the rapid change in the water column 13cplanktonic benthic offset during the glacial interglacial transition our results support the hypothesis that intermediate water plays an important role in propagating climatic changes from the polar regions to the tropics in this case climatic variations in the southern hemisphere may have led to the rapid ventilation of deep water and aaiw during termination 1 which contributed to the postglacial rise in atmospheric co2

response of calcareous nannofossil assemblages to paleoenvironmental changes through the mid pleistocene revolution at site 1090 southern ocean abstract quantitative study on calcareous nannofossil assemblages has been performed in high time resolution 2 3 kyr at the ocean drilling program site 1090 the location of this site in the southern ocean is crucial for the comprehension of thermohaline circulation and frontal boundary dynamics and for testing the employ of nannoflora as paleoceanographical tool the chronologically well constrained investigated record spans between marine isotope stage mis 35 and 15 through an interval of global paleoclimate and paleoceanographical modification also known as mid pleistocene revolution mpr measures of ecological shannon weaver diversity and paleoproductivity and dissolution indices together with spectral and wavelet analyses carried out on the acquired time series provide valuable information for interpretation of data in terms of paleoecology and paleoceanography assemblages are mainly represented by dominant small gephyrocapsa common calcidiscus leptoporus s l coccolithus pelagicus s l gephyrocapsa 4 5 5 m the extinct pseudoemiliania lacunosa and reticulofenestra spp r asanoi and reticulofenestra sp morphotypes discriminated within calcidiscus leptoporus s l and coccolithus pelagicus s l reveal that they may have had different ecological preferences during pleistocene with respect to the present the composition and fluctuation in nannofossil assemblage and their comparison with the available sea surface temperature sst and c org curves suggest a primary ecological response to paleoenvironmental changes relationships to different surface water features and boundary dynamics as well as to different efficiencies and motions of the intermediate and deep water masses have been inferred a more northward position of subantarctic front saf during most of the early pleistocene record has been highlighted based on assemblage composition characterised by common calcidiscus leptoporus s l coccolithus pelagicus s l medium gephyrocapsa 4 5 5 m and by the rarity or absence of umbilicosphaera spp rhabdosphaera spp pontosphaera spp oolithotus fragilis exceptions are the more intense interglacials mis 31 17 and probably mis 15 when a southward displacement of frontal system occurred coincident with peaks in abundance of helicosphaera spp and syracosphaera spp higher nutrient content and more dynamic conditions occurred between mis 32 and mis 25 in relation to shallower location of nutrient rich antarctic intermediate water aaiw core and to reduction of glacial interglacial variability a nannofossil barren interval is coincident with the known stagnation of south atlantic deep water circulation during mis 24 when north atlantic deep water nadw was reduced or suppressed and an enhanced northward deep penetration of the more corrosive circumpolar deep water cpdw took place an event of strong instability in nutricline dynamics characterised the transition mis 23 22 as suggested by sharp fluctuations in paleoproductivity proxies linked to major changes in oceanographic circulation and to the first distinct increase of larger ice volumes at this time from mis 21 upward the nannofossil variations seem to be primarily controlled by glacial interglacial cyclicity and temperature fluctuations the cyclic fluctuation recognised in nannofossil abundance seems to be linked to orbitally forced climatic variation primarily to the obliquity periodicity recorded in the patterns of c leptoporus intermediate 5 8 m and c pelagicus pelagicus 6 10 m however no obvious and linear relations may be always observed between nannoflora fluctuation and milankovitch parameters suggesting more complex and unclear relationships between nannofossils and environmental change

the cretaceous opening of the northeast atlantic abstract a continuous rift system from the rockall trough through the faeroe shetland channel and the more and voring basins up to the tromso and bear island basins is inferred to have developed during the cretaceous oceanic crust was generated in its southern part but its width presumably decreases toward the northeast and probably ends in the more basin this rift spreading system requires a clockwise rotation of greenland and the rockall plateau during the cretaceous around a rotation pole at its northern end at about 74 n 21 e north of the pole of rotation crustal shortening must have occurred this compression is interpreted to have taken place on west spitsbergen the orogeny there is reinterpreted as 1 1 a cretaceous folding phase 2 2 a phase of overthrusting at the paleocene eocene boundary and 3 3 an extensional faulting phase during the oligocene this plate tectonic model can resolve a series of problems in the northeast atlantic region the connection of the caledonian fronts of scotland and greenland can be easily established the northern ends of the west shetland basin and the north sea graben the unusual depth of the more and voring basins the marked obliqueness of structural trends between the northeast greenland shelf and the norwegian shelf the fact that the western hammerfest basin was intersected by a younger north south trending graben which formed the deep tromso basin all these enigmatic observations can be explained by the cretaceous rift system

effects of mountains and ice sheets on global ocean circulation abstractthe impact of mountains and ice sheets on the large scale circulation of the world s oceans is investigated in a series of simulations with a new coupled ocean atmosphere model oregon state university university of victoria model osuvic in which the height of orography is scaled from 1 5 times the actual height at t42 resolution to 0 no mountains the results suggest that the effects of mountains and ice sheets on the buoyancy and momentum transfer from the atmosphere to the surface ocean determine the present pattern of deep ocean circulation higher mountains reduce water vapor transport from the pacific and indian oceans into the atlantic ocean and contribute to increased decreased salinities and enhanced reduced deep water formation and meridional overturning circulation in the atlantic pacific orographic effects also lead to the observed interhemispheric asymmetry of midlatitude zonal wind stress the presence of the antarctic ice sheet cools winter air temperatures by more th

the mechanism for antarctic intermediate water renewal in a world ocean model abstract realistic representation of the low salinity tongue of antarctic intermediate water aaiw has been achieved in a coarse resolution ocean general circulation model the authors find that this water mass is not generated by direct subduction of surface water near the polar front instead the renewal process is concentrated in the southeast pacific ocean off southern chile the outflow of the east australian current progressively cools by heat loss to the atmosphere and freshens by assimilation of polar water carried north by the surface ekman drift during its slow movement across the south pacific toward the aaiw formation zone further deep warm advection near chile enables more convective overturn resulting in very deep mixed layers from which aaiw is fed into the south pacific and also into the malvinas current along with this isolated region of aaiw renewal the model relies on alongisopycnal mixing of fresh surface water from the polar front to capture a realistic circumpolar tongue

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

oceanic carbon and water masses during the mystery interval a model data comparison study 1 the mystery interval 17 5 14 5 ka bp is characterized by a large decline in atmospheric 14c synchronous with an increase in atmospheric co2 the most widely accepted hypothesis to explain these observed shifts involves the existence of an isolated old ocean carbon reservoir that was subsequently ventilated here we use the uvic earth system climate model to locate a potential carbon rich and 14c depleted water mass under 17 5 ka bp boundary conditions we then investigate two mechanisms for the potential ventilation of such a reservoir namely the weakening of the north atlantic meridional overturning due to iceberg calving and latitudinal shifts in southern hemisphere westerlies shw due to southern hemispheric warming we find that simulations derived from an equilibrium state forced with present day shw and moderate north atlantic deep water nadw formation are in better agreement with atmospheric and ocean 14c reconstructions than simulations derived from an equilibrium state forced with a northward shifted shw belt resulting in a shut down of the atlantic meridional overturning and formation of north pacific deep water for simulations with present day shw the oldest water masses are found in the north pacific although the southern ocean cannot be ruled out as a potential mystery reservoir according to our simulations the strength of atlantic overturning is the dominant mechanism in increasing the ocean atmosphere carbon flux while shifting shw results in a rearrangement of deep ocean carbon largely between the atlantic and pacific basins in our best case scenario the model can account for 58 of the atmospheric co2 increase and 48 of the atmospheric 14c decline while the rate of ventilation and the age of ventilated water masses are comparable with observations the ventilation in the model could not be sustained long enough to account for the full excursion seen in paleodata

semi adiabatic model of the deep stratification and meridional overturning abstractan analytical model of the atlantic deep stratification and meridional overturning circulation is presented that illustrates the dynamic coupling between the southern ocean and the midlatitude gyres the model expressed here in terms of the two and a half layer framework predicts the stratification and meridional transport as a function of the mechanical and thermodynamic forcing at the sea surface the approach is based on the classical elements of large scale circulation theory ideal thermocline inertial western boundary currents and eddy controlled antarctic circumpolar current acc models which are combined to produce a consistent three dimensional view of the global overturning the analytical tractability is achieved by assuming and subsequently verifying that the pattern of circulation in the model is largely controlled by adiabatic processes the time mean and eddy induced isopycnal advection of buoyancy the mean stratification of the lower thermocline is determined by the surface fo

coherent response of antarctic intermediate water and atlantic meridional overturning circulation during the last deglaciation reconciling contrasting neodymium isotope reconstructions from the tropical atlantic antarctic intermediate water aaiw plays important roles in the global climate system and the global ocean nutrient and carbon cycles however it is unclear how aaiw responds to global climate changes in particular neodymium isotopic composition end reconstructions from different locations from the tropical atlantic have led to a debate on the relationship between northward penetration of aaiw into the tropical atlantic and the atlantic meridional overturning circulation amoc variability during the last deglaciation we resolve this controversy by studying the transient oceanic evolution during the last deglaciation using a neodymium enabled ocean model our results suggest a coherent response of aaiw and amoc when amoc weakens the northward penetration and transport of aaiw decreases while its depth and thickness increase our study highlights that as part of the return flow of the north atlantic deep water nadw the northward penetration of aaiw in the atlantic is determined predominately by amoc intensity moreover the inconsistency among different tropical atlantic end reconstructions is reconciled by considering their corresponding core locations and depths which were influenced by different water masses in the past the very radiogenic water from the bottom of the gulf of mexico and the caribbean sea which was previously overlooked in the interpretations of deglacial end variability can be transported to shallow layers during active amoc and modulates end in the tropical atlantic changes in the aaiw core depth must also be considered thus interpretation of end reconstructions from the tropical atlantic is more complicated than suggested in previous studies

sensitivity of the southern ocean overturning circulation to surface buoyancy forcing 1 the sensitivity of the southern ocean overturning to altered surface buoyancy forcing is investigated in a series of eddy permitting idealised simulations the modelled response indicates that heat and freshwater fluxes in the southern hemisphere mid latitudes may play a significant role in setting the strength of the overturning circulation enhanced buoyancy fluxes act to increase the meridional overturning up to a limit approaching the wind driven ekman transport the sensitivity of the overturning to surface buoyancy forcing is strongly dependent on the relative locations of the wind stress profile buoyancy forcing and upwelling region the numerical simulations provide support for the hypothesis that changes in upwelling during deglaciations may have been driven by changes in heat and freshwater fluxes instead of or in addition to changes in wind stress

high latitude salinity effects and interhemispheric thermohaline circulations a general circulation model for the ocean is used to investigate the interaction between the global scale thermohaline circulation and the salinity distribution it is shown that an equatorially asymmetric circulation can be maintained even under equatorially symmetric basin geometry and surface forcing multiple equilibrium solutions are obtained for the same forcing by perturbing the high latitude salinity field in an otherwise equatorially symmetric initial condition the timescale of the transition from the symmetric circulation to an asymmetric circulation depends critically on the sign of the initial salinity perturbation

the potential of organic walled dinoflagellate cysts for the reconstruction of past sea surface conditions in the southern ocean abstract in this study we investigate the potential of organic walled dinoflagellate cysts dinocysts as tools for quantifying past sea surface temperatures sst in the southern ocean for this purpose a dinocyst reference dataset has been formed based on 138 surface sediment samples from different circum antarctic environments the dinocyst assemblages of these samples are composed of phototrophic gonyaulacoid and heterotrophic protoperidinioid species that provide a broad spectrum of palaeoenvironmental information the relationship between the environmental parameters in the upper water column and the dinocyst distribution patterns of individual species has been established using the statistical method of canonical correspondence analysis cca among the variables tested summer sst appeared to correspond to the maximum variance represented in the dataset to establish quantitative summer sst reconstructions a modern analogue technique mat has been performed on data from three late quaternary dinocyst records recovered from locations adjacent to prominent oceanic fronts in the atlantic sector of the southern ocean these dinocyst time series exhibit periodic changes in the dinocyst assemblage during the last two glacial interglacial cycles during glacial conditions the relative abundance of protoperidinioid cysts was highest whereas interglacial conditions are characterised by generally lower cyst concentrations and increased relative abundance of gonyaulacoid cysts the mat palaeotemperature estimates show trends in summer sst changes following the global oxygen isotope signal and a strong correlation with past temperatures of the last 140 000 years based on other proxies however by comparing the dinocyst results to quantitative estimates of summer ssts based on diatoms radiolarians and foraminifer derived stable isotope records it can be shown that in several core intervals the dinocyst based summer ssts appeared to be extremely high in these intervals the dinocyst record seems to be highly influenced by selective degradation leading to unusual temperature ranges and to unrealistic palaeotemperatures we used the selective degradation index kt index to determine those intervals that have been biased by selective degradation in order to correct the palaeotemperature estimates we show that after correction the dinocyst based ssts correspond reasonably well with other palaeotemperature estimates for this region supporting the great potential of dinoflagellate cysts as a basis for quantitative palaeoenvironmental studies

seismic volcanostratigraphy of the norwegian margin constraints on tectonomagmatic break up processes voluminous volcanism characterized early tertiary continental break up on the mid norwegian continental margin the distribution of the associated extrusive rocks derived from seismic volcanostratigraphy and potential field data interpretation allows us to divide the more voring and lofoten vesteralen margins into five segments the central more margin and the northern voring margin show combinations of volcanic seismic facies units that are characteristic for typical rifted volcanic margins the lofoten vesteralen margin the southern voring margin and the area near the jan mayen fracture zone show volcanic seismic facies units that are related to small volume submarine volcanism the distribution of subaerial and submarine deposits indicates variations of subsidence along the margin vertical movements on the mid norwegian margin were primarily controlled by the amount of magmatic crustal thickening because both the amount of dynamic uplift by the icelandic mantle plume and the amount of subsidence due to crustal stretching were fairly constant along the margin thus subaerial deposits indicate a large amount of magmatic crustal thickening and an associated reduction in isostatic subsidence whereas submarine deposits indicate little magmatic thickening and earlier subsidence from the distribution of volcanic seismic facies units we infer two main reasons for the different amounts of crustal thickening 1 a general northward decrease of magmatism due to increasing distance from the hot spot and 2 subdued volcanism near the jan mayen fracture zone as a result of lateral lithospheric heat transport and cooling of the magmatic source region furthermore we interpret small lateral variations in the distribution of volcanic seismic facies units such as two sets of inner seaward dipping reflectors on the central voring margin as indications of crustal fragmentation

cosmic ray production rates of 10be and 26al in quartz from glacially polished rocks we have studied the concentrations of 10be and 26al in quartz crystals extracted from glacially polished granitic surfaces from the sierra nevada range these surfaces were identified with the glacial advance during the tioga period 11 000 years ago our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in sio2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43 8 44 6 n and altitudes 2 1 3 6 km the estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0 75 with respect to the horizontal the principal uncertainty arises due to the lack of a precise date for the glacial retreat event about 10 the 26al 10be ratio at production 6 0 is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides production rates of 10be and 26al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies

north atlantic climate responses to perturbations in antarctic intermediate water recent observations suggest antarctic intermediate water aaiw properties are changing the impact of such variations is explored using idealised perturbation experiments with a coupled climate model hadcm3 aaiw properties are altered between 10 and 20 s in the south atlantic maintaining constant potential density the perturbed aaiw remains subsurface in the south atlantic but as it moves northwards it surfaces and interacts with the atmosphere leading to density anomalies due to heat exchanges for a cooler fresher aaiw there is a significant decrease in the mean north atlantic sea surface temperature sst of up to 1 c during years 51 100 in the north atlantic current region there are persistent cold anomalies from 2 000 m depth to the surface and in the overlying atmosphere atmospheric surface pressure increases over the mid latitude atlantic and precipitation decreases over northwest africa and southwest europe surface heat flux anomalies show that these impacts are caused by changes in the ocean rather than atmospheric forcing the sst response is associated with significant changes in the atlantic meridional overturning circulation moc after 50 years there is a decrease in the moc that persists for the remainder of the simulation resulting from changes in the column averaged density difference between 30 s and 60 n rather than showing a linear response a warmer saltier aaiw also leads to a decreased moc strength for years 51 100 and resulting cooling in the north atlantic the non linearity can be attributed to opposing density responses as the perturbed water masses interact with the atmosphere

rapid changes in meridional advection of southern ocean intermediate waters to the tropical pacific during the last 30 kyr the southern ocean is increasingly recognized as a key player in the general ocean thermohaline circulation and the global climate system during glacial interglacial transitions in particular the advection of southern ocean intermediate waters soiw like antarctic intermediate water and subantarctic mode water to the eastern equatorial pacific eep through a so called oceanic tunnelling mechanism is an important means for rapid transfer of climatic signals such as heat fresh water salt and chemical species from high to low latitudes however information on how intermediate water advection rates changed in the past and particularly during deglaciations is fragmentary we present new results for nd isotopes end in cleaned foraminifera shells neogloboquadrina dutertrei for the last 30 kyr at odp site 1240 in the eep n dutertrei preferentially dwells in the lower thermocline at the core of the equatorial undercurrent euc and the end variability over time provides a record of the changes in the end of the euc through mixing models we show that the euc record is primarily controlled by changes in the volume transport of intermediate waters and not by southern ocean end changes southern ocean signals in the euc are stronger during colder intervals younger dryas last glacial maximum and heinrich stadials 1 and 2 in agreement with tropical atlantic intermediate water records in addition covariations between n dutertrei d 13 c molecular biomarkers and diatom productivity at site 1240 confirm the intermediate water route as an important mechanism for the transfer of climate signals from high to low latitudes changes in the soiw chemistry during the deglaciation are likely linked to the upwelling of old deep waters in the southern ocean and subsequent export as intermediate waters which are coeval with the atmospheric co2 rise moreover a comparison of multiple proxy records for the last 30 kyr indicates a latitudinal shift and or a change in the convection depth of intermediate waters in the southern ocean prior to the onset of the deglaciation

cenozoic sequence stratigraphy of the central and northern north sea basin tectonic development sediment distribution and provenance areas the cenozoic succession in the central and northern north sea has been investigated to establish a regional sequence stratigraphic framework changes in sediment distribution indicate a complex pattern of regional vertical movements along older palaeozoic and mesozoic structures in cenozoic times these vertical movements mainly related to tectonic processes along the continental margin to the north and north west were responsible for the generation and removal of provenance areas for sediments delivered to the north sea basin through cenozoic time during the early through middle palaeogene sediments were mainly sourced from areas in the west and from the atlantic margin in the north uplift in the northern north sea in the late eocene was in the earliest oligocene followed by marked basin subsidence and tectonic uplift of southern norway a similar pattern of tectonic movements resulted in subaerial exposure of the northern north sea in the early miocene followed by significant tectonic subsidence in the basin and along the atlantic margin in late miocene pliocene times at the same time southern norway was uplifted and became a major sediment source after the miocene pliocene subsidence uplift events glacial processes in southern norway and fluvial processes in a large drainage area east and south east of the north sea were responsible for the main sediment influx to the north sea the cenozoic depositional sequences in the north sea developed in close interaction with regional tectonic movements and changes in provenance areas tectonic movements in north west europe overprinted global sea level changes so that the generation of depositional sequences and sequence boundaries apparently occurred independently of the rate of eustatic sea level change