effective medium theories for fluid saturated materials with aligned cracks there is general agreement between different theories giving expressions for the overall properties of materials with dry aligned cracks if the number density of cracks is small there is also very fair agreement for fluid filled isolated cracks however there are considerable differences between two separate theories for fluid filled cracks with equant porosity comparison with recently published experimental data on synthetic sandstones gives a good fit with theory for dry samples however although the crack number density in the laboratory sample is such that first order theory is unlikely to apply expressions correct to second order in the number density provide a worse fit it also appears that the ratio of wavelength to crack size is not sufficiently great for any detailed comparison with effective medium theories which are valid only when this ratio is large the data show dispersion effects for dry cracks and scattering neither of which will occur at sufficiently long wavelengths data from the water saturated samples indicate that the effect of equant porosity is significant although the two theories differ strongly as to just how significant once again and in spite of the reservations mentioned above a reasonable fit between theory and observation can be shown

the origin of teleseismic pn waves multiple crustal scattering of upper mantle whispering gallery phases 1 teleseismic pn arrivals with an extensive coda are observed to offsets beyond 3000 km along the peaceful nuclear explosion seismic profiles quartz and ruby which were recorded in the western part of the former soviet union we interpret these arrivals as multiply reflected sub moho refractions that travel over large distances due to a positive vertical upper mantle velocity gradient which is characteristic for the study area analysis of the observed data shows that the teleseismic pn and its coda contain significant amounts of energy at all frequencies between 0 and 10 hz our modeling results show that crustal scattering may fully explain the teleseismic pn coda contrary to another published model with upper mantle heterogeneity in the 35 130 km depth range these conclusions are based on two dimensional viscoelastic finite difference seismic wave field simulations in 2000 km long and 250 km deep models of the crust and upper mantle the computationally demanding calculations are facilitated by the use of supercomputer systems our preferred model of crustal scattering is consistent with the results of other high resolution wide angle and normal incidence seismic investigations of the crust and upper mantle they show pronounced reflectivity of the lower crust above an almost transparent uppermost mantle down to 80 100 km depth our model includes a high vertical velocity gradient in the upper mantle based on the results from seismic refraction studies we suggest that the teleseismic pn wave travels as an upper mantle whispering gallery phase and that the origin of its long coda is crustal scattering

oxygen isotope analyses and pleistocene ice volumes abstract the oxygen isotope record from fossil foraminifera in deep sea sediments is commonly used as a proxy for global ice volume the linkage between 18 o and ice volume however is probably nonlinear we have developed a simple numerical model of the isotopic response of the oceans to ice volume change the major features it simulates are 1 the changing mean isotopic composition of snow as a function of ice volume colder snow temperatures forced by climate change and higher elevation accumulation areas imply more negative mean 18 o 2 the nonequilibrium isotopic composition of ice sheets the past history of an ice sheet is integrated into its mean isotopic composition which introduces a lag of isotopic ice volume i e the measured 18 o record scaled to ice volume units behind true ice volume 3 selective preservation of isotopically more negative colder higher latitude ice this geographic effect can selectively amplify or dampen the isotopic response to the ice volume signal we illustrate the response of our model to simple hypothetical ice volume transitions of ice growth and ice decay sensitivity tests are illustrated for all model parameters the results suggest that oxygen isotope records reproduce the general patterns of ice volume change fairly accurately the foraminiferal isotope record however may misrepresent the true amplitude of the ice volume signal and lag true ice volume by 1000 to 3000 yr

pn anisotropy in mesozoic western pacific lithosphere pn is the high frequency scattered p phase guided for great distances within the old oceanic lithosphere two arrays of ocean bottom seismometers were deployed on old 150 160 ma seafloor in the northwestern pacific south of shatsky rise for the pacific lithosphere anisotropy and thickness experiment we use pn phases from 403 earthquakes during the 1 year of deployment to measure apparent velocities across the arrays each array was deployed on a separate limb of a magnetic bight formed near a fast spreading ridge ridge ridge triple junction using high frequency waves 5 10 hz we look at variations of pn velocities as a function of azimuth in the western array we find pn anisotropy with velocities ranging from 8 7 km s in the back azimuth direction of 310 to 7 7 km s at 350 in the eastern array the velocity ranges from 8 5 km s in back azimuth direction of 210 to 7 7 km s at 260 and 310 we observe rapid velocity changes with azimuth in the both arrays requiring sinusoidal variations of roughly equal amplitude as a function of both 2 and 4 which is not expected for the orthorhombic symmetry of olivine or orthopyroxene the fastest directions on the two limbs are roughly orthogonal to each other suggesting the dominance of fossil anisotropy but the fast directions of the 2 components are skewed counterclockwise from the spreading directions we speculate that the rapid azimuthal variations may be caused by vertical stratification with changing anisotropy with depth in the oceanic lithosphere

a tropical mechanism for northern hemisphere deglaciation 1 we investigate the role of the tropics in the melting and reforming of the laurentide ice sheet on glacial timescales using an atmospheric general circulation model it is found that warming of tropical sea surface temperatures ssts from glacial boundary conditions as observed at the end of glacial periods causes a large increase in summer temperatures centered over the ice sheet forming regions of canada this highlatitude response to tropical change is due to relatively small changes in the circulation of the extratropical atmosphere which lead to changes in the vertical profiles of temperature and moisture in the extratropical atmosphere the maximum perturbation in the summer radiative balance over the laurentide ice sheet 25 w m 2 due to the changes in the local atmospheric water vapor inventory is much larger than that induced by glacial to interglacial changes in atmospheric co2 these changes via an atmospheric bridge between the tropics and extratropics represent a mechanism for deglaciations which is consistent with timing constraints in contrast a cold perturbation to tropical sst for interglacial boundary conditions results in only very small changes in the delivery of water vapor to the laurentide region and therefore almost no cooling over the canadian region this implies that tropical ssts could play a more important role in melting ice sheets in the northern hemisphere than in reforming them possibly providing a mechanism which could help to explain the rapidity of deglaciation relative to glacial inception

synthetic pn and sn phases and the frequency dependence of q of oceanic lithosphere the oceanic lithosphere is an extremely efficient waveguide for high frequency seismic energy in particular the propagation of the regional to teleseismic oceanic pn and sn phases is largely controlled by properties of the oceanic plates the shallow velocity gradient in the sub moho lithosphere results in a nearly linear travel time curve for these oceanic phases and an onset velocity near the material velocity of the uppermost mantle the confinement of pn sn to the lithosphere imposes a constraint on the maximum range that a normally refracted wave can be observed the rapid disappearance of sn and the discontinuous drop in pn sn group velocity beyond a critical distance dependent upon the local thickness of the lithosphere are interpreted as a shadowing effect of the low q asthenosphere wave number integration was used to compute complete synthetic seismograms for a model of oceanic lithosphere the results were compared to data collected during the 1983 ngendei seismic experiment in the southwest pacific the pn sn coda is successfully modeled as a sum of leaky organ pipe modes in the sediment layer and oceanic water column while scattering is present to some degree it is not required to explain the long duration and complicated nature of the pn sn wave trains the presence of extremely high frequencies in pn sn phases and the greater efficiency of sn than pn propagation are interpreted in terms of an absorption band rheology a shorter high frequency relaxation time for p waves than for s waves results in a rheology with the property that q q at low frequency while q q at high frequency consistent with the teleseismic pn sn observations the absorption band model is to viewed as only an approximation to the true frequency dependence of q in the oceanic lithosphere for which analytic expressions for the material dispersion have been developed

anisotropy in the oceanic lithosphere of the north western pacific basin summary a series of long range explosion seismological experiments has been conducted by the use of specially designed ocean bottom seismographs obss in the western pacific obs studies of apparent velocity measurements by the use of natural earthquakes have also been made the experiments have made clear that large scale p wave anisotropy exists in the entire thickness of the oceanic lithosphere the existence of the large scale anisotropy in the oceanic lithosphere has been demonstrated for the first time by seismic body wave studies previously anisotropy had been found only in the uppermost oceanic mantle in the eastern pacific the azimuth of the maximum velocity 8 6km s l is about 155 clockwise from north the direction is perpendicular to the magnetic lineation of the region however the direction differs from the direction of the present plate motion by about 30 so it appears that the anisotropy has been frozen at least since the change of the plate motion that occurred 40myr ago the frozen anisotropy should set important constraints on the mechanical properties of the lithosphere such as the viscosity and temperature of the lower lithosphere

shallow mantle anisotropy beneath the juan de fuca plate the anisotropic fabric of the oceanic mantle lithosphere is often assumed to parallel paleo relative plate motion rpm however we find evidence that this assumption is invalid beneath the juan de fuca jdf plate using travel times of seismic energy propagating through the topmost mantle we find that the fast direction of p wave propagation is rotated 18 3 counterclockwise to the paleo spreading direction and strikes between pacific jdf relative and jdf absolute plate motion apm the mean mantle velocity is 7 85 0 02 km s with 4 6 0 4 anisotropy synthesis of the plate averaged pn anisotropy signal with measurements of pn anisotropy beneath the jdf ridge and sks splits across the jdf plate suggest that the anisotropic structure of the topmost mantle continues to evolve away from the spreading center to more closely align with apm we infer that the oceanic mantle lithosphere may record the influence of both paleo rpm and paleo apm

the sensitivity of the climate response to the magnitude and location of freshwater forcing last glacial maximum experiments abstract proxy records indicate that the locations and magnitudes of freshwater forcing to the atlantic ocean basin as iceberg discharges into the high latitude north atlantic laurentide meltwater input to the gulf of mexico or meltwater diversion to the north atlantic via the st lawrence river and other eastern outlets may have influenced the north atlantic thermohaline circulation and global climate we have performed last glacial maximum lgm simulations with the ncar community climate system model ccsm3 in which the magnitude of the freshwater forcing has been varied from 0 1 to 1 sv and inserted either into the subpolar north atlantic ocean or the gulf of mexico in these glacial freshening experiments the less dense freshwater provides a lid on the ocean water below suppressing ocean convection and interaction with the atmosphere above and reducing the atlantic meridional overturning circulation amoc this is the case whether the freshwater is added directly to the area of convection south of greenland or transported there by the subtropical and subpolar gyres when added to the gulf of mexico the amoc reduction is less for the smaller freshwater forcings but is not linear with the size of the freshwater perturbation the recovery of the amoc from a slow state is 200 years for the 0 1 sv experiment and 500 years for the 1 sv experiment for glacial climates with large northern hemisphere ice sheets and reduced greenhouse gases the cold subpolar north atlantic is primed to respond rapidly and dramatically to freshwater that is either directly dumped into this region or after being advected from the gulf of mexico greenland temperatures cool by 6 8 c in all the experiments with little sensitivity to the magnitude location or duration of the freshwater forcing but exhibiting large seasonality sea ice is important for explaining the responses the northern hemisphere high latitudes are slow to recover antarctica and the southern ocean show a bipolar response with warming and reduced sea ice this warming continues after the cessation of the freshwater forcing and shows a dependence on the duration of the freshwater forcing equatorward of the expanded sea ice the simulated temperature and salinity anomalies are sensitive to the amount of colder and fresher waters that are advected out of the subpolar north atlantic in the tropical atlantic the recovery of the intertropical convergence zone itcz from its more southerly position during the freshwater forcing is much more rapid than the recovery of the amoc and is more related to the recovery of low latitude surface temperatures than greenland temperature or sea ice these results have implications for using proxy records as indirect measures of the amoc

on the nature of p n pn phases are observed along many refraction seismic profiles and are common in earthquake records their velocities usually range from 7 8 to 8 2 km s 1 classical ray theory used to interpret these observations implies a positive upper mantle velocity gradient however a wide spread positive velocity gradient in the lithospheric mantle is not expected from petrological and petrophysical data laboratory velocity measurements at elevated temparatures and pressures suggest positive velocity gradients only for very low heat flow values 40 mw m 2 higher heat flow causes negative gradients consequently petrological models of the upper mantle would restrict pn observations to precambrian shields and old platforms contrary to observations we overcome this contradiction by considering media that contain random velocity fluctuations superimposed on positive or negative velocity gradients in both cases these structures generate pn phases by wide angle scattered waves short wavelength random velocity fluctuations of only 0 5 1 superimposed on negative velocity gradients are sufficient for generating pn phases consequently this implies that an observed pn wave does not necessitate a positive upper mantle velocity gradient for a peridotitic upper mantle fluctuations of this size can be explained by slightly varying the relative proportions of its mineralogical constituents anisotropy is likely to contribute to the inferred fluctuations

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

stratification of anisotropy in the pacific upper mantle 1 on the basis of the use of broadband 25 150 s rayleigh wave group speeds to estimate the 2 component of azimuthal anisotropy we present evidence for a stratification of anisotropy in the uppermost mantle at large scales across the pacific basin we confirm previous surface wave studies that established that the fast axis directions of azimuthal anisotropy for intermediate and long period rayleigh waves approximately align with present day plate motions at shorter periods 25 50 s however fast axes align nearer to the paleospreading or fossil spreading direction which differs from present day plate motion in the old pacific these observations as well as observations of the age dependence of the amplitude of azimuthal anisotropy imply that azimuthal anisotropy in the pacific upper lithosphere 100 km depth is fixed or fossilized on average reflecting the strain conditions extant during the early evolution of the lithosphere rather than the current ambient flow direction in the deeper lithosphere and asthenosphere anisotropic fast axis directions align nearer to present day plate motions apparently having reoriented to conform to the current conditions of mantle flow the mechanism of anisotropy stratification remains unclear but observations are consistent with the anisotropy of the shallow lithosphere being fixed because the shear flows that can produce dislocation creep and a change in anisotropy will occur at increasing depths as the plate ages

comment on possible forms of seismic anisotropy of the uppermost mantle under oceans by george e backus over 15 years ago backus 1965 demonstrated approximate equations for the velocity variations of body waves in weakly anisotropic solids the p wave equation is widely quoted aki and richards 1980 and many others as modeling p wave variations over all planes in all anisotropic solids as there appears to be revival of interest in anisotropy it is worthwhile to note a minor limitation to the application of the p wave equation and to draw attention to some similar linear equations for shear waves which combined with the p wave equation are very successful in modeling the velocity variations of anisotropic solids

constraining past seawater δ18o and temperature records developed from foraminiferal geochemistry paired measurements of magnesium to calcium ratios mg ca and the stable oxygen isotopic composition 18o in foraminifera have significantly advanced our knowledge of the climate system by providing information on past temperature and seawater 18o 18osw a proxy for salinity and ice volume however multiple sources of uncertainty exist in transferring these downcore geochemical data into quantitative paleoclimate reconstructions here we develop a computational toolkit entitled paleo seawater uncertainty solver psu solver that performs bootstrap monte carlo simulations to constrain these various sources of uncertainty psu solver calculates temperature and 18osw and their respective confidence intervals using an iterative approach with user defined errors calibrations and sea level curves our probabilistic approach yields reduced uncertainty constraints compared to theoretical considerations and commonly used propagation exercises we demonstrate the applicability of psu solver for published records covering three timescales the late holocene the last deglaciation and the last glacial period we show that the influence of salinity on mg ca can considerably alter the structure and amplitude of change in the resulting reconstruction and can impact the interpretation of paleoceanographic time series we also highlight the sensitivity of the records to various inputs of sea level curves transfer functions and uncertainty constraints psu solver offers an expeditious yet rigorous approach to test the robustness of past climate variability inferred from paired mg ca 18o measurements

ecological controls on the shell geochemistry of pink and white globigerinoides ruber in the northern gulf of mexico implications for paleoceanographic reconstruction abstract we evaluate the relationship between foraminiferal test size and shell geochemistry 13c 18o and mg ca for two of the most commonly used planktonic foraminifers for paleoceanographic reconstruction in the subtropical atlantic ocean the pink and white varieties of globigerinoides ruber geochemical analyses were performed on foraminifera from modern core top samples of high accumulation rate basins in the northern gulf of mexico mg ca analysis indicates a positive relationship with test size increasing by 1 1 mmol mol 2 5 c from the smallest 150 212 m to largest 500 m size fractions of g ruber pink but with no significant relationship in g ruber white in comparison oxygen isotope data indicate a negative relationship with test size decreasing by 0 6 across the size range of both pink and white g ruber the observed increase in mg ca and decrease in 18o are consistent with an increase in calcification temperature of 0 7 c per 100 m increase in test size suggesting differences in the seasonal and or depth distribution among size fractions overall these results stress the necessity for using a consistent size fraction in downcore paleoceanographic studies in addition we compare downcore records of 18o and mg ca from pink and white g ruber in a decadal resolution 1000 year sedimentary record from the pigmy basin based on this comparison we conclude that pink g ruber is calcifying in warmer waters than co occurring white g ruber suggesting differences in the relative seasonal distribution and depth habitat of the two varieties

hydrological changes in eastern europe during the last 40 000 yr inferred from biomarkers in black sea sediments the black sea is connected to a large drainage area including the european russian plain part of the alps and southeastern europe to study the hydrological changes in this basin over the last 40 000 years we measured specific terrigenous biomarkers for wetland vegetation in well dated sediments from the northwestern black sea spanning the last glacial period lacustrine phase and the holocene marine phase low abundances of these biomarkers are observed during the north atlantic ice melting and cooling events known as heinrich events 4 to 2 the last glacial maximum and the younger dryas event increased biomarker inputs characterize the mild climate phases known as dansgaard oeschger interstadials the bolling allerod and preboreal warmings indicating increased erosion due to permafrost degradation higher primary productivity and or wetland extension in the drainage basin the final retreat of the fennoscandian ice sheet from the russian plain occurs during the early part of heinrich event 1 and is characterized by increased biomarker concentrations in a typical series of four deglacial clay layers for the last glacial period the correspondence in timing between the biomarker records and the atmospheric ch4 record from ice cores suggests an important ch4 contribution due to boreal permafrost thawing and wetland emission

upper mantle and crustal seismic structure beneath the northwestern pacific basin using a seafloor borehole broadband seismometer and ocean bottom seismometers abstract in august 2000 a seafloor borehole broadband seismometer wp 2 was installed in the northwestern pacific basin and seismic experiments with ocean bottom seismometers obss a single channel seismic streamer and airguns were performed the wp 2 continued to record data autonomously and a total of 436 days of broadband seismic records were eventually retrieved reflecting the low noise environment many teleseismic events were recorded shallow seismic velocity models just below the obss were derived by the tau p p method for the airgun obs data and the single channel seismic data the crustal structure was estimated from the obss and wp 2 data by forward modeling using a two dimensional ray tracing method the results of the seismic surveys show that the crustal seismic structure around wp 2 is laterally homogeneous and corresponds to typical oceanic crust the uppermost mantle exhibits seismic anisotropy the velocity variations are about 5 for p waves and about 3 5 for s waves and the fast direction appears to be perpendicular to the magnetic lineations travel times of earthquakes recorded by the wp 2 and the previous seismological studies suggest that the lower part of the lithosphere has greater anisotropy than the uppermost mantle to explain late first arrivals from the earthquakes that occurred in the slow direction with epicentral distances between 1600 and 2200 km a low velocity zone below a depth of 30 km and a rapid increase of velocity at a depth of 210 km are inferred receiver function analysis of 16 events with a high signal to noise s n ratio from the wp 2 data was performed and discontinuities were estimated at depths of 416 and 666 km these discontinuity depths are consistent with those of the average beneath the ocean

azimuthal seismic anisotropy of 70 ma pacific plate upper mantle author posting american geophysical union 2019 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research solid earth 124 2 2019 1889 1909 doi 10 1029 2018jb016451

longshot experiments to study velocity anisotropy in the oceanic lithosphere of the northwestern pacific abstract several long range explosion seismology experiments have been conducted in the northwestern pacific basin where one of the oldest oceanic lithospheres is postulated to exist the experiments were conducted from 1974 to 1980 highly sensitive ocean bottom seismographs which had been developed for longshot experiments were used the lengths of the profiles ranged from 1000 to 1800 km and the directions were chosen to provide wide azimuthal coverage one of the aims of this series of experiments was to test the existence of velocity anisotropy on a large regional scale the results show that the oceanic lithosphere has anisotropy wherein the velocity changes by 4 7 the anisotropy extends from a depth of at least 40 to 140 km beneath the sea bottom however the magnitude of the anisotropy may vary with depth the azimuth of the maximum velocity is 150 160 clockwise from north and coincides with the fossil direction of spreading of the pacific plate whereas it differs from the present direction of plate motion by 30 the azimuth does not seem to depend on depth in the direction of maximum velocity the lithosphere is basically two layered 8 0 8 2 and 8 6 km s 1 the depth of the interface is 50 60 km beneath the sea floor

seasonal laurentide ice sheet melting during the mystery interval 17 5 14 5 ka the last deglaciation in the northern hemisphere was interrupted by two major stadials the so called mystery interval 17 5 14 5 ka and the younger dryas 12 9 11 7 ka during these events the north atlantic region was marked by cold surface conditions yet simultaneous glacier and snowline retreat rerouting of laurentide ice sheet meltwater from the gulf of mexico to an eastern or northern spillway may have reduced meridional overturning circulation at the onset of the younger dryas however this hypothesis has not been tested for the mystery interval paired mg ca and 18 o measurements on foraminifera from laminated orca basin sediments in the gulf of mexico constrained by 35 14 c dates document the timing of meltwater input with subcentennial resolution isolating the 18 o of seawater termed 18 o gom reveals three major melting episodes from ca 17 5 ka until 12 9 ka followed by a rapid cessation consistent with meltwater rerouting at the onset of the younger dryas conversely inferred meltwater fl ow to the gulf of mexico during the mystery interval does not support a simple routing event but is consistent with glacier and snowline retreat we suggest that summer melting of northern hemisphere ice sheets during this stadial may have been an important mechanism for enhanced winter sea ice formation hypercold winter conditions and enhanced seasonality in the north atlantic region

deglacial abrupt climate change in the atlantic warm pool a gulf of mexico perspective 1 during the last deglaciation greenland ice core and north atlantic sediment records exhibit multiple abrupt climate events including the younger dryas cold episode 12 9 11 7 ka however evidence for the presence of the younger dryas in the gulf of mexico gom and the relationship between gom sea surface temperature sst and high latitude climate change is less clear we present new mg ca sst records from two varieties of the planktonic foraminifer globigerinoides ruber white and pink to assess northern gom sst history from approximately 18 4 10 8 ka thirty five accelerator mass spectrometry ams 14c dates from orca basin core md02 2550 provide excellent age control and document high sedimentation rates 40 cm kyr g ruber white and pink mg ca sst data exhibit increases 4 6 0 6 c and 2 2 0 5 c respectively from at least 17 8 16 6 ka with nearly decadal resolution that are early relative to the onset of the bolling allerod interstadial moreover g ruber white sst decreases at 16 0 14 7 ka 1 0 0 5 c and 12 8 11 6 ka 2 4 0 6 c correlate to the oldest and younger dryas in greenland and cariaco basin the g ruber pink sst record which reflects differences in seasonality and or depth habitat is often not in phase with g ruber white and closely resembles antarctic air temperature records overall it appears that orca basin sst records follow antarctic air temperature early in the deglacial sequence and exhibit enhanced seasonality during greenland stadials

changing amounts and sources of moisture in the u s southwest since the last glacial maximum in response to global climate change abstract the u s southwest has a limited water supply and is predicted to become drier in the 21st century an improved understanding of factors controlling moisture sources and availability is aided by reconstruction of past responses to global climate change new stable isotope and growth rate records for a central texas speleothem indicate a strong influence of gulf of mexico gom moisture and increased precipitation from 15 5 to 13 5 ka which includes the majority of the bolling allerod warming ba 14 7 12 9 ka coeval speleothem records from 900 and 1200 km to the west allow reconstruction of regional moisture sources and atmospheric circulation the combined isotope and growth rate time series indicates 1 increased gom moisture input during the majority of the ba producing greater precipitation in texas and new mexico and 2 a retreat of gom moisture during younger dryas cooling 12 9 11 5 ka reducing precipitation these results portray how late pleistocene atmospheric circulation and moisture distribution in this region responded to global changes providing information to improve models of future climate

pn anisotropy beneath the south island of new zealand and implications for distributed deformation in continental lithosphere pn travel times from regional earthquakes recorded both by stations on new zealand and by ocean bottom seismographs deployed offshore indicate anisotropy in the uppermost mantle beneath the region the largest anisotropy of 8 2 1 lies beneath the deforming part of the south island to just off its west coast a zone roughly 100 200 km wide the fastest propagation is aligned n60 e 3 essentially parallel to the largely strike slip relative plate motion since 20 ma also n60 e the magnitude of anisotropy decreases abruptly northwest and southeast of this zone and on the southeast side of the island the orientation of fastest propagation is between n32 w and n s the n60 e orientation of fast propagation is consistent with finite strain within the uppermost part of the mantle lithosphere if the measured 850 km of displacement of the pacific plate past the australia plate is spread over a region with a width of 100 200 km the agreement of this orientation of fast propagation with the orientation or relative plate motion suggests the possibility of but does not require some dynamic recrystallization in rock as cold as 500 800 c where peierls creep seems to be the likely deformation mechanism such a strain distribution matches deformation of a thin viscous sheet that obeys a constitutive relationship of the form e n where e is the average strain rate and is the operative deviatoric stress with an average value of n 3 10 presumably the nw se fast propagation in the region southeast of the island results from strain that precedes the cenozoic deformation that has shaped the island

on the fidelity of shell derived δ18oseawater estimates article i nfo 18 oseawater a robustproxyforsurfacesalinity we assessthe fidelity of shell derived 18 oseawater estimates for the surface dwelling foraminifer globigerinoides ruber white using an atlantic meridional coretop transect spanning basin scale temperature and salinity gradients shell derived and observed 18 oseawater values are well correlated r 2 0 77 but a large systematic bias is observed shell mg ca ratios are significantly elevated above values expected from observed and isotopic calcification temperatures in the saline subtropical gyres of both hemispheres this temperature independent shell mg ca ratio variability termed excess mg ca i s highly correlated with surface salinity r 2 0 77 and the observed salinity dependence 27 4 is much higher than indicated by culture studies 6 2 our coretop data are used to develop new atlantic basin temperature and salinity calibration equations that are accurate 1 1 c and 0 20 respectively precise r 2 0 82 and 0 81 respectively and verifiable using previously published data these results are valid for the relatively high salinities of the subtropical atlantic 35 5 37 3 we discover that inclusion of other published data from lower salinity regions b35 indicates little or no excess mg ca taken together these results point to a strongly non linear positive salinity effect on shell mg ca ratios that significantly affects the accuracy of sst and 18

routing of meltwater from the laurentide ice sheet during the younger dryas cold episode rooth1 proposed that the younger dryas cold episode which chilled the north atlantic region from 11 000 to 10 000 yr bp was initiated by a diversion of meltwater from the mississippi drainage to the st lawrence drainage system the link between these events is postulated to be a turnoff during the younger dryas cold episode of the north atlantic s conveyor belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the north atlantic region2 this turnoff is attributed to a reduction in surface water salinity and hence also in density of the waters in the region where north atlantic deep water nadw now forms here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from orca basin core en32 pc4 which reveal a significant reduction in meltwater flow through the mississippi river to the gulf of mexico from about 11 200 to 10 000 radiocarbon years ago this finding is consistent with the record for lake agassiz which indicates that the meltwater from the southwestern margin of the laurentide ice sheet was diverted to the northern atlantic ocean through the st lawrence valley during the interval from 11 000 to 10 000 years before present yr bp

transient simulation of last deglaciation with a new mechanism for bolling allerod warming we conducted the first synchronously coupled atmosphere ocean general circulation model simulation from the last glacial maximum to the bolling allerod ba warming our model reproduces several major features of the deglacial climate evolution suggesting a good agreement in climate sensitivity between the model and observations in particular our model simulates the abrupt ba warming as a transient response of the atlantic meridional overturning circulation amoc to a sudden termination of freshwater discharge to the north atlantic before the ba in contrast to previous mechanisms that invoke amoc multiple equilibrium and southern hemisphere climate forcing we propose that the ba transition is caused by the superposition of climatic responses to the transient co 2 forcing the amoc recovery from heinrich event 1 and an amoc overshoot

mantle deformation during slow seafloor spreading constrained by observations of seismic anisotropy in the western atlantic during mantle flow at mid ocean ridges viscous shear deformation imparts a structural fabric that remains in the lithosphere as it cools preserving a record of ridge processes over time this fabric can be detected by seismic imaging as it produces an azimuthal anisotropy in the velocity of seismic waves that propagate in the shallow mantle using data from a novel seismic refraction survey in the western atlantic we found that the maximum p wave velocity in the upper 10 km of mantle lithosphere formed at the slow spreading mid atlantic ridge is parallel to the paleo spreading direction consistent with viscous shear deformation dominated by corner flow the magnitude of the p wave anisotropy is 3 4f0 3 approximately one half that found in lithosphere formed at faster spreading rates in the pacific weaker anisotropy in the atlantic suggests that more pervasive conductive cooling at slow spreading ridges increases the proportion of localized brittle deformation in the mantle lithosphere thereby limiting the degree of viscous deformation by scaling our field observations to laboratory experiments we estimate that total viscous strain during slow seafloor spreading is of order 0 5 as opposed to 1 2 for the fast spreading case finally our travel time observations display an azimuthal asymmetry that can be interpreted in one of two ways either the average elastic tensor in this region is not orthorhombic as is commonly assumed or the underlying shear fabric is rotated 158 relative to fossil spreading d 2004 elsevier b v all rights reserved

glacial to holocene terrigenous organic matter input to sediments from orca basin gulf of mexico a combined optical and biomarker approach in this study we assessed changes in the contribution of terrigenous organic matter om to the gulf of mexico over the course of the last deglaciation the last 25 kyr to this end we combined optical kerogen analyses with bulk sedimentary biomarker and compound specific carbon isotope analyses samples were obtained from core md02 2550 from orca basin 2249 m water depth at 26 56 77n 91 20 74w with temporal resolution ranging from multi decadal to millennial scale depending on the proxy all proxies confirmed larger terrigenous input during glacial times compared to the holocene in addition the kerogen analyses suggest that much of the glacial om is reworked at least 50 of spores and pollen grains and 40 of dinoflagellate cysts the holocene sediments in contrast contain mainly marine om which is exceptionally well preserved during the deglaciation terrigenous input was generally high due to large meltwater fluxes whereby discrepancies between different proxies call for additional influences such as the change in distance to the river mouth local productivity changes and hydrodynamic particle sorting it is possible that kerogen particles and the terrigenous biomarkers studied here represent distinct pools of land derived om with inputs varying independently

on the seasonal sea ice cover of the sea of okhotsk free of ice 4 months of the year the sea of okhotsk begins to freeze over in a few coastal locations in november attains its maximum ice coverage usually in mid march and by june retains ice in only a few coastal regions this sequence is well recorded by satellite imagery from the electrically scanning microwave radiometer esmr on board nimbus 5 four years of esmr data reveal significant interannual variations in the maximum ice cover and in the smoothness of the growth and retreat periods of the years 1973 1976 the heaviest ice cover occurred in 1973 and the growth period most interrupted by temporary ice edge retreats was in 1974 comparison of the general features of the cycle of sea ice distribution with basic oceanographic factors shows that ice forms first in cold shallow low salinity waters and then seems to drift in a direction approximating the okhotsk kuril current system two basic patterns of ice edge positioning which occur frequently in the esmr data are identified as a rectangular structure and a wedge structure each of these is strongly correlated with the bathymetry of the region and with the known current system suggesting that convective depth and ocean currents play an important role in determining ice patterns certain major interannual variations in the ice extents are accounted for by interannual contrasts in the atmospheric pressure fields

rayleigh wave phase velocities in the pacific with implications for azimuthal anisotropy and lateral heterogeneities summary the lateral distribution of fundamental mode rayleigh wave phase velocities in the pacific has been calculated in order to determine the variation in velocity as a function of the age of the oceanic plate the importance of azimuthal anisotropy and the presence of secondary lateral heterogeneities the data set used in this study consists of phase velocity measurements in the period range 20 125 s for 178 paths traversing the pacific both the pure path and spherical harmonic inversion techniques are used in this investigation with an emphasis placed on determining the effectiveness and resolving power of these methods in calculating the velocity distribution in an oceanic regime the first order velocity age relation which dominantly characterizes surface wave dispersion in an oceanic environment can be adequately modelled by either the pure path or spherical harmonic techniques azimuthal anisotropy is the dominant second order effect and is best developed in regions less than 80 myr in age in this age zone the faster direction of wave propagation is either in the direction of fossil seafloor spreading or in the direction of present day absolute plate motion in the western pacific there is a correlation with paleo relative motion fossil seafloor spreading for periods less than 50 s lateral velocity heterogeneities which are not related to the age of the oceanic plate are found by pure path inversions and are correlated with regions associated with shallow residual depth anomalies this effect slow velocities with respect to the velocity age model monotonically decreases as a function of period and is indicative of a shallow origin age independent anomalies were also modelled by the application of the pure path and spherical harmonic methods in a sequential inversion

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

permafrost conditions in the mediterranean region since the last glaciation the relatively warm climate conditions prevailing today in the mediterranean region limit cold geomorphological processes only to the highest mountain environments however climate variability during the late pleistocene and holocene has led to significant spatio temporal variations of the glacial and periglacial domain in these mountains including permafrost conditions here we examine the distribution and evolution of permafrost in the mediterranean region considering five time periods last glaciation deglaciation holocene little ice age lia and present day the distribution of inactive permafrost derived features as well as sedimentary records indicates that the elevation limit of permafrost during the last glaciation was between 1000 m and even 2000 m lower than present permafrost was also widespread in non glaciated slopes above the snowline forming rock glaciers and block streams as well as meter sized stone circles in relatively flat summit areas as in most of the northern hemisphere the onset of deglaciation in the mediterranean region started around 19 20 ka the ice free terrain left by retreating glaciers was subject to paraglacial activity and intense periglacial processes under permafrost conditions many rock glaciers protalus lobes and block streams formed in these recently deglaciated environments though most of them became gradually inactive as temperatures kept rising especially those at lower altitudes following the younger dryas glacial advance the early holocene saw the last massive deglaciation in mediterranean mountains accompanied by a progressive shift of permafrost conditions to higher elevations it is unlikely that air temperatures recorded in mediterranean mountains during the holocene favoured the existence of widespread permafrost regimes with the only exception of the highest massifs exceeding 2500 3000 m lia colder climate promoted a minor glacial advance and the spatial expansion of permafrost with the development of new protalus lobes and rock glaciers in the highest massifs finally post lia warming has led to glacial retreat disappearance enhanced paraglacial activity shift of periglacial processes to higher elevations degradation of alpine permafrost along with geoecological changes

surface wave tomography for the pacific ocean incorporating seafloor seismic observations and plate thermal evolution abstract using broadband seismic waveforms recorded on the seafloor by more than 200 broadband ocean bottom seismometers as well as those on land we measured the phase speed dispersions of love and rayleigh waves up to the 4th higher mode to determine the three dimensional radially anisotropic shear wave speed structure in the upper mantle beneath the pacific ocean the fastest anomalies at depths shallower than 100 km were located beneath southeast of the shatsky rise and strong radial anisotropy was located in the central pacific at depths of 100 200 km the isotropic shear wave speed structures showed age dependence from the age bin averaged shear wave speed profiles and a half space cooling model we estimated the thermo speed relationship for the pacific plate to construct a reference age dependent shear wave speed model which was further used as an initial model for tomography iteration deviation maps in the pacific ocean from the reference model indicated that large negative residuals which may be due to partial melting anelasticity and or added heat from mantle plumes were located along the ridges and beneath hotspots and that large positive residuals were found beneath the northwestern pacific ocean the use of an age dependent reference model as well as the incorporation of obs data greatly improves the accuracy of local phase speed estimates in tomography as evident from a direct comparison with in situ array measurements

phasing of deglacial warming and laurentide ice sheet meltwater in the gulf of mexico evidence is emerging that the tropical climate system played a major role in global climate change during the last deglaciation however existing studies show that deglacial warming was asyn chronous across the tropical band complicating the identification of causal mechanisms the orca basin in the northern gulf of mexico is ideally located to record subtropical atlantic sea surface temperature sst warming in relation to meltwater input from the laurentide ice sheet paired d 18 o and mg ca data on the planktonic foraminifer globigerinoides ruber from core en32 pc6 are used to separate deglacial changes in sst and d 18 o of sea water sst as calculated from mg ca data increased by 3 8c from ca 17 2 to 15 5 ka in association with heinrich event 1 and was not in phase with greenland air temperature subtracting tem perature effects from d 18 o values in g ruber reveals two excur sions representing laurentide meltwater input to the gulf of mex ico one of 1 5 from ca 16 1 to 15 6 ka and a second major spike of 2 5 from ca 15 2 to 13 0 ka that encompassed melt water pulse 1a and peaked ca 13 8 ka during the bolling allerod conversion to salinity through the use of a laurentide meltwater end member of 225 indicates that near surface salinity de creased by 2 4 during these spikes these results suggest that gulf of mexico sst warming preceded peak laurentide ice sheet decay and the bolling allerod interval by 2 k y and that heat was retained in the subtropical atlantic during heinrich event 1 consistent with modulation of deglacial climate by thermohaline circulation

anisotropic propagation of p and s waves in the western pacific lithosphere summary apparent velocities for oceanic p and s phases from nw circum pacific earthquakes recorded on an ocean bottom hydrophone array near wake island vary with azimuth of approach the apparent velocity data from events in the mariana islands are fast whereas velocities of events between the izu bonin islands and the kurils are slower for data showing slow apparent velocities the path average group velocity exceeds the local apparent velocity the azimuthal variation of oceanic p and s apparent velocity is consistent with azimuthal anisotropy and the slow direction of propagation is nearly parallel to the strike of local magnetic lineations propagation paths that parallel the local magnetic lineations near the obh array are more perpendicular to lineations in the region of the nw pacific basin where the paths originate the observation of path group velocity faster than local apparent velocity for oceanic p and s is qualitatively consistent with a re orientation of anisotropy as manifested in the magnetic lineations along the propagation path between source and receiver results suggest that the western pacific lithosphere is azimuthally anisotropic for p and s wave propagation

timing of the east antrim coastal readvance phase relationships between lowland irish and upland scottish ice sheets during the last glacial termination a submillennial ice readvance from upland centres of ice dispersal in west central scotland into northeastern ireland post dates the retreat of lowland irish ice immediately after the killard point stadial max 16 5 cal ka bp the dimensions of this southerly and westerly ice sheet readvance on the margins of the north channel are reconstructed from subglacial bedform patterns subglacial tectonic deformation of tertiary lignite and glacigenic sediment limiting moraines outwash and glacial stratigraphy morainic ridges at rams island and sandy bay on the eastern margin of the lough neagh basin which mark this ice limit are perpendicular to a well defined field of subglacial bedforms across east county antrim at the ice readvance limits glacial lakes impounded in the lagan valley were partially infilled with subaqueous outwash known as the malone sands this water body drained south along the dundonald comber gap spillway providing sediment which formed extensive late glacial marine terraces at the northern end of strangford lough around 15 15 5 cal ka bp the east antrim coastal readvance is part of a much more extensive readvance southwards along the north channel and adjacent lowlands associated with ice sheet reorganisation and ice sheet growth in west central scotland it is now termed the north channel readvance and may be similar in age to the wester ross readvance moraines in northwestern scotland

the oxygen isotopic composition of seawater during the last glacial maximum high resolution oxygen and hydrogen isotope measurements were made on pore fluids from deep sea sediments from sites in the north and south atlantic the data provide direct measurements of changes in the isotopic composition of bottom waters during the last glacial maximum lgm results from ocean drilling program odp site 981 in the north atlantic currently bathed in north atlantic deep water nadw reproduces previous results from the ceara and bermuda rises constraining the glacial interglacial change in 18 o of the deep atlantic to be 0 7 0 8 results from site 984 which is located north of site 981 and at a shallower water depth yield a similar value 0 8 providing insight into the properties of glacial north atlantic intermediate water gnaiw sites from odp leg 177 in the south atlantic span the modern boundary between northern and southern sources of deep water data from the northern site 1088 yield a similar result to sites in the tropical and north atlantic 0 7 at the southern site 1093 located south of the polar front the change is substantially larger 1 1 representing the change in 18 o of southern source waters since the lgm these results confirm previous estimates that the global average change in 18 o of seawater is 1 0 0 1 hydrogen isotopes measured on pore fluids from three sites are consistent with the oxygen isotopes from these locations giving further support to these results at all sites studied the temperature of the deep ocean during the lgm calculated by combining the pore fluid results with oxygen isotope data from benthic foraminifera was within 1 c of the freezing point of seawater

the chronology climate and confusion of the moorhead phase of glacial lake agassiz new results from the ojata beach north dakota usa rapid drainage of glacial lake agassiz to the north atlantic ocean has been implicated as a triggering mechanism for the younger dryas yd cold event 13 11 6 ka cal a key component to this hypothesis is the interpretation that the ojata beach of lake agassiz in north dakota usa formed during a regression of the lake at the beginning of the yd this paper reviews the chronological data for the lowwater moorhead phase presents new radiocarbon and optically stimulated luminescence osl ages for the ojata beach and utilizes plant and insect macrofossils to reconstruct the paleoenvironmental conditions during the moorhead phase in eastern north dakota the integrated analysis of the geochronologic data emphasizes the need to distinguish between in situ and reworked plant macrofossils new ages obtained stratigraphically below the ojata beach sediments and reinterpreted chronologic data for the moorhead phase suggest that contrary to previous interpretations the ojata beach is a record of transgression of the lake in the later part of the yd and the oldest in situ minimum age for the moorhead phase is 10 47775 ka 14 c bp paleoenvironmental analysis indicates that a

p and s wave anisotropy of a synthetic sandstone with controlled crack geometryl1 a method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix a synthetic sandstone was manufactured from sand cemented with an epoxy glue the cracks of known geometry were introduced into the material in the manufacturing stage by emplacing thin metallic discs in the sand epoxy matrix these discs were chemically leached out of the consolidated porous sandstone acoustic anisotropy and shear wave splitting were observed in the synthetic sandstones for the dry sample the observed angular dependence of the p and s wave velocities at 100 khz compares well qualitatively with the theoretical models of hudson and of thomsen quantitatively however the experimental data fits hudson s model better for the case of a saturated sample the experimental results are in excellent agreement with thomsen s model hudson s model on the other hand predicts a different angular dependence for p waves this demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process the results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry

did north atlantic overturning halt 17 000 years ago models indicate that a complete shutdown of deep and intermediate water production is a possible consequence of extreme climate conditions in the northern north atlantic and the high ratio of 231pa to 230th on bermuda rise is evidence that this might have happened 17 ka during heinrich event 1 h1 however new radiocarbon data from bivalves that lived at 4 6 km on the bermuda rise during h1 lead to a different conclusion the bivalve data do indeed indicate ventilation of the deep western north atlantic was suppressed during h1 but not as much as it was during the last glacial maximum we propose that high diatom flux to the bermuda rise during h1 is at least in part responsible for increased 231pa 230th at that time although we cannot say for sure why opal production was so high in a gyre center location at that time increased leakage of silica rich waters from the southern ocean to the north atlantic is one possibility

seismic anisotropy in the upper mantle 2 predictions for current plate boundary flow models 1 the anisotropic seismic structure due to flow induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary the orientation distributions of olivine enstatite aggregates evolve along streamlines of each flow model based on each grains plastic response to the local stress strain field the effective elastic tensor for these textured aggregates provides predictions of p wave anisotropy and shear wave splitting throughout the model space p and s travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model comparison of predicted fast p direction for our method versus a finite strain based estimate shows that agreement is acceptable for much of the model space but notable differences occur in regions up to several tens of kilometers in size two dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle the effect of flow in the third dimension is addressed in a few calculations for one comparison of flow in the mantle wedge at a subduction zone the introduction of trench parallel flow causes significant changes in the predicted patterns of p wave anisotropy magnitude more than orientation and sks splitting

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

azimuthal anisotropy of the pacific region azimuthal anisotropy is the dependence of local seismic properties on the azimuth of propagation we present the azimuthally anisotropic component of a 3d sv velocity model for the pacific ocean derived from the waveform modeling of over 56 000 multi mode rayleigh waves followed by a simultaneous inversion for isotropic and azimuthally anisotropic vsv structure the isotropic vsv model is discussed in a previous paper a maggi e debayle k priestley g barruol multi mode surface waveform tomography of the pacific ocean a close look at the lithospheric cooling signature geophys j int 166 3 2006 doi 10 1111 j 1365 246x 2006 03037 x the azimuthal anisotropy we find is consistent with the lattice preferred orientation model lpo the hypothesis of anisotropy generation in the earth s mantle by preferential alignment of anisotropic crystals in response to the shear strains induced by mantle flow at lithospheric depths we find good agreement between fast azimuthal anisotropy orientations and ridge spreading directions recorded by sea floor magnetic anomalies at asthenospheric depths we find a strong correlation between fast azimuthal anisotropy orientations and the directions of current plate motions we observe perturbations in the pattern of seismic anisotropy close to pacific hot spots that are consistent with the predictions of numerical models of lpo generation in plume disturbed plate motion driven mantle flow these observations suggest that perturbations in the patterns of azimuthal anisotropy may provide indirect evidence for plume like upwelling in the mantle

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

deglacial ocean warming and marine margin retreat of the cordilleran ice sheet in the north pacific ocean abstract a new high resolution planktonic foraminiferal mg ca based ocean temperature record has been generated for deep sea core md02 2496 sited offshore of vancouver island western canada during the last deglaciation 21 12 ka the relationship between cordilleran ice sheet cis retreat and changing regional ocean temperatures has been reconstructed through glaciomarine sediments in md02 2496 that capture tidewater glacier response to surface ocean thermal forcing at cis maximum extent the marine margin of the ice sheet advanced onto the continental shelf during this interval ocean temperatures recorded by surface dwelling globigerina bulloides remained a relatively constant 7 5 c while subsurface dwelling neogloboquadrina pachyderma s recorded temperatures of 5 c these ocean temperatures were sufficiently warm to induce significant melt along the tidewater ice terminus similar to modern alaskan tidewater glacial systems during the deglacial retreat of the cis the n pachyderma temperature record shows two distinct warming steps of 2 and 2 5 c between 17 2 16 and 15 5 14 ka respectively coincident with ice rafting events from the cis while g bulloides records an 3 c warming from 15 to 14 ka we hypothesize that submarine melting resulting from relatively warm ocean temperatures was an important process driving ice removal from cis tidewater glaciers during the initial stages of deglaciation

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments

the role of seasonality in abrupt climate change abstract a case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the north atlantic region during the last glaciation and into the holocene the primary evidence comes from mismatches between mean annual temperatures from greenland ice cores in comparison with snowline changes in east greenland northern europe and north america the most likely explanation is a shutdown or reduction in strength of the conveyor this allows the spread of winter sea ice across the north atlantic thus causing the northern region to experience much colder winters because they mimic the greenland temperature rather than the snowline signal changes in the atlantic intertropical convergence zone and the asian monsoon may also share a winter linkage with greenland thus the paleoclimate record is consistent with the notion that a huge continental sector of the northern hemisphere stretching from greenland to asia was close to an extreme winter threshold during much of the last glaciation winter climate crossed this threshold repeatedly with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

magnetic anomaly lineations from late jurassic to early cretaceous in the west central pacific ocean summary late jurassic to early cretaceous mesozoic magnetic anomaly lineations the phoenix magellan mid pacific mountains hawaiian and japanese lineation sets with fracture zones in the west central pacific ocean were identified more comprehensively than in any previous studies we fixed 2100 positions of identified magnetic anomalies based on magnetic data of 283 cruise tracks two remarkable fracture zones the phoenix and central pacific fracture zones were mapped and newly named our newly identified lineations from mlon to mo around the mid pacific mountains which belong to the hawaiian lineation set illustrated that the sea floor south of the mid pacific mountains has the same age as that of the north 132 118 ma our analysis of skewness parameters revealed that the older part of the phoenix set m17 m29 has skewness different from that of the younger part ml m14 implying an effect of magnetic overprints by the cretaceous volcanism it was confirmed that the spreading rate of the mesozoic pacific spreading system was the fastest in the world in the mesozoic a drastic change in spreading rates occurred simultaneously at the period between chrons m21 and m20 149 5 148 5 ma in all the mesozoic pacific spreading systems the event appears to be synchronous with events in other oceans such as the mesozoic atlantic and indian oceans

freshwater forcing of abrupt climate change during the last glaciation large millennial scale fluctuations of the southern margin of the north american laurentide ice sheet occurred during the last deglaciation when the margin was located between about 43 and 49 n fluctuations of the ice margin triggered episodic increases in the flux of freshwater to the north atlantic by rerouting continental runoff from the mississippi river drainage to the hudson or st lawrence rivers we found that periods of increased freshwater flow to the north atlantic occurred at the same time as reductions in the formation of north atlantic deep water thus providing a mechanism for observed climate variability that may be generally characteristic of times of intermediate global ice volume