patterns of family extinction depend on definition and geological timescale the recent claim1 of an approximate 26 myr periodicity in the pattern of mass species extinction over the past 250 myr has already stimulated much astrophysical speculation and debate on causal mechanisms2 13 however as shown here the evidence for that claim is strongly contingent on arbitrary decisions concerning the absolute dating of stratigraphical boundaries14 the culling of the database and the definition of what is mass extinction as opposed to background extinction this evidence becomes insufficient under other plausible geological timescales and other acceptable definitions of mass extinction analysis of the nonculled database shows that the reliability of identification of mass extinctions and their timing is at present extremely limited it also suggests that the apparent periodicity of mass extinctions results from stochastic processes

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

changes in ocean heat content caused by wave induced mixing in a high resolution ocean model abstract a parameterization of turbulent mixing from unbroken surface waves is included in a 16 yr simulation within a high resolution ocean circulation model mom5 this surface wave mixing sw

alluvial architecture of the holocene rhine meuse delta the netherlands ancient fluvial successions often act as hydrocarbon reservoirs sub surface data on the alluvial architecture of fluvial successions are often incomplete and modelling is performed to reconstruct the stratigraphy however all alluvial architecture models suffer from the scarcity of field data to test and calibrate them the purposes of this study were to quantify the alluvial architecture of the holocene rhine meuse delta the netherlands and to determine spatio temporal trends in the architecture five north south orientated cross sections perpendicular to the general flow direction were compiled for the fluvial dominated part of the delta these sections were used to calculate the width thickness ratios of fluvial sandbodies sbw sbt and the proportions of channel belt deposits cdp clastic overbank deposits odp and organic material op in the succession furthermore the connectedness ratio cr between channel belts was calculated for each cross section distinct spatial and temporal trends in the alluvial architecture were found sbw sbt ratios decrease by a factor of ca 4 in a downstream direction cdp decreases from ca 0 7 upstream to ca 0 3 downstream op increases from less than 0 05 in the upstream part of the delta to more than 0 25 in the downstream delta odp is approximately constant 0 4 cr is ca 0 25 upstream which is approximately two times larger than in the downstream part of the delta furthermore cdp in the downstream rhine meuse delta increases after 3000 cal yr bp these trends are attributed to variations in available accommodation space floodplain geometry and channel belt size for instance channel belts tend to narrow in a downstream direction which reduces sbw sbt cdp and cr tectonics cause local deviations in the general architectural trends in addition the positive correlation between avulsion frequency and the ratio of local to regional aggradation rate probably influenced alluvial architecture in the rhine meuse delta the rhine meuse data set can be a great resource when developing more sophisticated models for alluvial architecture simulation which eventually could lead to better characterizations of hydrocarbon reservoirs to aid such usage of the rhine meuse data set constraints for relevant parameters are provided at the end of the paper

deglacial warming of the northeastern atlantic ocean correlation with the paleoclimatic evolution of the european continent abstract isotopic micropaleontologic and pollen analyses of deep sea cores from the bay of biscay and the northeastern atlantic ocean show that the deglacial warming of this oceanic area was closely correlated with the paleoclimatic evolution of the adjacent european continent temperatures were at least as warm as those of today in the bay of biscay between 13 300 and 11 000 b p coinciding with the combined bolling allerod warm continental events a major spread of polar water occurred between 11 000 and 10 000 b p during this event which coincides with the younger dryas continental cold event marine temperatures were almost as low as those of the last glacial maximum the final deglacial warming of the norteastern atlatntic ocean occurred during the following 3000 yr

holocene climatic changes in the western mediterranean from south east france to south east spain abstract holocene climatic changes along coastal regions from south east france to south east spain were studied using pollen ratios comparing modern pollen rain vegetation and climate along selected transects from the atlantic ocean to the mediterranean we obtained threshold values of two different ratios corresponding to the different climatic conditions along the transects these pollen ratios and threshold values were employed to characterize the holocene climatic changes from nine mediterranean coastal sites the results were compared with data from marine and continental pollen sequences distributed in the western mediterranean basin and with additional regional data independent of human activity lake level fluctuations alpine glacier advance and retreat chronology 14 c anomaly and cooling phases in eastern france and central europe the role of anthropogenic activities and climate on the changes in vegetation is discussed six major changes in vegetation cover were identified they correspond to aridification phases that occurred around 9500 9000 yr bp 10 900 9700 cal bp 7500 7000 yr bp 8400 7600 cal bp 4500 4000 yr bp 5300 4200 cal bp 3700 3300 yr bp 4300 3400 cal bp 2600 1900 yr bp 2850 1730 cal bp and 1300 1000 yr bp 1300 750 cal bp these arid episodes were regional responses to more global climatic changes and determined the changes in the vegetation cover humans undoubtedly enhanced the vegetation changes but none the less had to adapt to these new climatic conditions

assessment of wave height distributions using an extensive field database abstract the present paper investigates the short term statistical distribution of wave heights specifically some of the most commonly applied wave height distributions are assessed using field measurements the latter comprise of wave radar observations from 10 different locations in the north sea and cover water depths between 7 7 m and 45 m in total the field database includes more than 200 million waves making it one of the largest of its kind in this water depth regime in using these data the accuracy of existing wave height distributions has been examined and guidance is provided concerning the best performing models and their domain of applicability additionally insights concerning the influence of key met ocean parameters are also provided taken together the results in this paper present an overview of the statistical behaviour of wave heights in finite water depths as observed in the field

modeling the trajectories of satellite tracked drifters in the adriatic sea during a summertime bora event 1 during the summertime bora episode of 22 25 june 1995 characterized by hazardous weather conditions with strong winds of over 20 m s 1 two satellite tracked drifters were present in the adriatic shelf area one was on the open sea south of the istrian peninsula while the other which measured the surface current of 70 cm s 1 was tracked along the western coastal strip an oceanographic model forced by the outputs of mesoscale meteorological models and by river discharges was applied to simulate drifter trajectories during the event process oriented studies performed to determine governing forcings for the drifter movements in different areas of the adriatic revealed that strong current along the western adriatic coast resulted from a combined influence of the wind stress and the river discharges the wind was responsible for the strong drifter movements while the baroclinic current due to the coastal buoyancy fluxes prevented the drifter from moving close to the shore and from colliding with the coast at the same time drifter movements in the open sea resulted from the wind action solely differences in the alongshore and offshore vertical density profiles primarily due to the river inflows caused 3 times stronger currents in the coastal area as compared to the currents in the open sea the proper formulation of the drag coefficient was crucial for the successful simulations of the drifter trajectories as simulations with increased values of the drag coefficient reproduced well the effects of the sea surface roughness and atmospheric instability above the sea

bed slope effects on turbulent wave boundary layers 2 comparison with skewness asymmetry and other effects 1 a numerical model solving incompressible reynolds averaged navier stokes equations combined with a two equation k model for turbulence closure is used to systematically compare the relative strength of bed shear stress quantities and boundary layer streaming under wave motions from four contributions believed to play a prominent role in cross shore boundary layer and sediment transport processes 1 converging diverging effects from bed slope 2 wave skewness 3 wave asymmetry and 4 waves combined with superposed negative currents intended to loosely represent for example return currents or undertow the effects from each of the four components are isolated and quantified using a standard set of bed shear stress quantities allowing their easy comparison for conditions representing large shallow water waves on steep slopes the results suggest that converging diverging effects from beach slope may make a significant onshore bed load contribution generally however the results suggest wave skewness in addition to conventional steady streaming as the most important onshore contribution outside the surf zone streaming induced within the wave boundary layer is also investigated for each component and skewness and asymmetry are demonstrated to promote largely negative streaming velocities consistent with earlier work for hydraulically smooth cases however a thin region of positive streaming is revealed in the viscous sublayer which is effectively absent in hydraulically rough conditions cross shore boundary layer dynamics are generally acknowledged to be a complicated combination of numerous competing physical processes and this work is among few to systematically compare a number of these it is hoped that the results will be useful in assessing which of the contributions considered will be most prominent for a given set of local wave and beach slope conditions

vertical structure of surface gravity waves propagating over a sloping seabed theory and field measurements 1 theoretical predictions of the vertical structure of wave motion over a sloping seabed are compared with field observations close to the bed in the nearshore zone of particular interest is the effect of the local slope on the magnitude and phase of the vertical velocity field measurements of near bed velocity profiles on a 2 bed slope were obtained using a coherent doppler profiler the surface elevation was measured by a colocated upward looking acoustic sounder results are presented from two intervals of different wave energy levels during a storm event for wave height water depth ratios smaller than 0 3 and for ursell numbers smaller than 0 6 the local comparisons of magnitude and phase between the vertical velocity and surface elevation measurements are in good agreement with linear theory for a sloping bed but differ greatly from that for a horizontal bottom especially in the lower water column the sloping bottom however has little effect on the horizontal velocity linear theory appears to adequately describe the transfer function between the surface elevation and the near bed velocities not only at the peak frequencies but also at their harmonics however in relatively shallow water the local transformations of free and forced waves at the harmonic frequencies are indistinguishable in the lower water column therefore given surface elevation measurements at a particular location which reflect the integrated effects of nonlinearities associated with wave shoaling the vertical structure of the third moments of velocity fields estimated from linear theory is in reasonable agreement with the observations both theory and observations show that the skewness and asymmetry of the vertical velocity are subject to significant bottom slope effects whereas those of horizontal velocity are not index terms 4211 oceanography general benthic boundary layers 4203 oceanography general analytical modeling 4219 oceanography general continental shelf processes 4546 oceanography physical nearshore processes 4560 oceanography physical surface waves and tides 1255 keywords shoaling waves bottom slope vertical structure skewness asymmetry boundary layers citation zou q a e hay and a j bowen vertical structure of surface gravity waves propagating over a sloping seabed theory and field measurements j geophys res 108 c8 3265 doi 10 1029 2002jc001432 2003

beginnings of olive cultivation in eastern spain in relation to holocene bioclimatic changes morphometric analyses show quantitative differences in anatomical characters of wood and charcoal between wild and cultivated olive samples from modern olive wood in eastern spain levante provide five distinctive anatomical criteria growth width ring vessel surface number of vessels per group vessel density and vulnerability ratio multivariate analysis shows that growth ring width and number of vessels per group are both significant criteria for discriminating between wild and cultivated olive moreover bioclimatic environments of wild olive thermomediterranean and mesomediterranean stages are distinguished by vessel density ancient olive charcoal from archaeological sites at valencia and alicante implies increasing aridification from the cardial neolithic to the roman period this pattern may reflect the onset of a mediterranean climate and human deforestation charcoal from cultivated specimens of early neolithic age shows that the olive tree is the earliest cultivated temperate fruit

application of artificial neural networks ann to high latitude dinocyst assemblages for the reconstruction of past sea surface conditions in arctic and sub arctic seas the artificial neural network ann method was applied to dinoflagellate cyst dinocyst assemblages to estimate palaeoceanographical conditions the ann method was adapted to three distinct data bases covering the northern north atlantic n 371 plus the arctic seas n 540 and the bering sea n 646 the relative abundance of 23 dinocyst taxa was calibrated against hydrographic variables sea surface temperature salinity and density in february and august and seasonal extent of sea ice cover using anns the estimation of hydrographical parameters based on an ann yields high coefficients of correlation between observations and reconstructions for each variable selected the validation tests performed on the different data bases suggest more accurate calibration at the scale of the north atlantic and arctic n 540 than on a multibasin scale i e when including the subpolar north pacific n 646 the ann calibrations and the modern analogue technique mat have been applied to two sequences from the northwest north atlantic spanning the past 25 000 yr for the purpose of comparison both approaches yielded similar results generally within the range of their respective uncertainties demonstrating their suitability the main discrepancies generally correspond to assemblages with poor modern analogues for which we have to admit a higher degree of uncertainties in the reconstruction whatever the approach used copyright 2001 john wiley sons ltd

evidence from crater ages for periodic impacts on the earth recent evidence has indicated that the impact of a comet or asteroid may have been responsible for mass extinction at the ends of both the cretaceous1 and the eocene2 4 quantitative analysis by raup and sepkoski5 showed that mass extinctions occur with a 26 myr period similar to the period seen in qualitative pelagic records by fischer and arthur6 to account for the possibility of periodic comet showers davis et al 7 proposed that such showers could be triggered by an unseen solar companion star as it passes through perihelion on a moderately eccentric orbit to test a prediction implicit in this model we examined records of large impact craters on the earth we report here that most of the craters occur in a 28 4 myr cycle within measurement errors this period and its phase are the same as those found in the fossil mass extinctions the probability that such agreement is accidental is 1 in 103

late glacial and holocene palaeoclimatic record from sierra de cebollera northern iberian range spain abstract the study of a pollen sequence obtained from a peat deposit in hoyos de iregua iberian range north central spain has been carried out revealing new information about the vegetation dynamics and climate changes since late glacial times pollen zones have been defined corresponding to successive phases in vegetation history during the end of the late wurm oldest dryas late glacial interstadial younger dryas and the holocene during the oldest dryas event there was a sparse vegetation cover dominated by herbaceous taxa with pinus as the unique arboreal taxon with some importance the vegetation landscape during the late glacial interstadial 13 000 11 000 yr bp corresponded to coniferous forest and mixed forests including the presence of evergreen quercus between the late glacial and the holocene a brief cooler and drier phase occurred the younger dryas event 11 000 10 000 yr bp during this period an important reduction of the tree masses characterized the evolution of the landscape the holocene began with a period of increasing temperature that resulted in an expansion of the arboreal elements which reached their highest representation during the holocene climatic optimum from 5060 90 bp a change to more arid conditions occurred favouring the development of the mediterranean vegetation within the late holocene clear evidence of the human impact on the environment are detected being more accentuated towards the uppermost levels

the effect of wave induced turbulence on the ocean mixed layer during tropical cyclones field observations on the australian north west shelf 1 field observations of water temperature on the australian north west shelf eastern indian ocean with the support of numerical simulations are used to demonstrate that the injection of turbulence generated by the wave orbital motion substantially contributes to the mixing of the upper ocean measurements also show that a considerable deepening of the mixed layer occurs during tropical cyclones when the production of wave induced turbulent kinetic energy overcomes the contribution of the current generated shear turbulence despite a significant contribution to the deepening of the mixed layer the effect of a background current and atmospheric forcing are not on their own capable of justifying the observed deepening of the mixed layer through most of the water column furthermore variations of a normally shallow mixed layer depth are observed within a relatively short timescale of approximately 10 hours after the intensification of wave activity and vanish soon after the decay of storm surface waves this rapid development tends also to exclude any significant contribution by wave breaking as small rates of vertical diffusivity for wave breaking induced turbulence would require longer timescales to influence the depth of the mixed layer

modern pollen climate calibration for southern south america abstract using the response surface technique a modern pollen climate calibration is presented for the mid latitudes in southern south america between latitudes 39 s and 44 s extending from the west coast in chile across the andes into the patagonian steppe region the climate parameters identified that relate to the present day pollen distribution of the nine most important pollen types nothofagus dombeyi type nothofagus obliqua type podocarpus weinmannia caldcluvia eucryphia type cupressaceae myrtaceae misodendrum and steppe types are summer precipitation summer and winter temperatures using the modern pollen climate data set three fossil pollen records mallin aguado and lago moreno east of the andes and caunahue west of the andes are interpreted in terms of these three seasonal climate parameters despite intervals of no analog fossil pollen assemblages the reconstruction shows similar climate trends on both sides of the andes during the last 21 000 cal yr bp 17 000 yr bp with summer precipitation as low or markedly lower than today prior to 17 000 cal yr bp 14 500 yr bp as high or higher than today between 15 000 and 12 500 cal yr bp 12 500 and 10 500 yr bp and again lower than today between 12 500 and 8000 cal yr bp 10 500 and 7200 yr bp summer and winter temperatures were low or high when summer precipitation was high or low

measurements of momentum and heat transfer across the air sea interface abstract this study makes direct measurements of turbulent fluxes in the mixed layer in order to close heat and momentum budgets across the air sea interface and to assess the ability of rigid boundary turbulence models to predict mean vertical gradients beneath the ocean s wavy surface observations were made at 20 hz at nominal depths of 2 2 and 1 7 m in 16 m of water a new method is developed to estimate the fluxes and the length scales of dominant flux carrying eddies from cospectra at frequencies below the wave band the results are compared to independent estimates of those quantities with good agreement between the two sets of estimates the observed temperature gradients were smaller than predicted by standard rigid boundary closure models consistent with the suggestion that wave breaking and langmuir circulation increase turbulent diffusivity in the upper ocean similarly the monin obukhov stability function h was smaller in the authors measurements than the stability functions used in rig

how dating uncertainties affect the detection of periodicity in extinctions and craters abstract claims of a highly significant detection of a period of 25 30 myr in the records of major extinction events and large craters have prompted us to examine how dating errors degrade a periodic signal we ran monte carlo simulations in which we placed eight events at equal time intervals and then displaced each event by a random error chosen from a normal distribution if the r m s error is greater than about 13 of the period the periodic data is usually no longer distinguishable from random data at the 90 confidence level at present the r m s dating errors in extinction episodes and craters over the last 250 myr are at least 6 myr or 20 of a 25 to 30 myr period we conclude that even if a periodicity were present it would not normally be detected at a statistically significant level by the tests used in the literature the apparent periodicity is probably due to a statistical fluke or subjective bias

on the wave and current interaction with a rippled seabed in the coastal ocean bottom boundary layer interactions of currents and waves with a rippled seabed in the inner part of the coastal ocean bottom boundary layer are studied using particle image velocimetry adv and bottom roughness measurements mean velocity profiles collapse with appropriate scaling in the log layer but vary substantially in the roughness sublayer when wave induced motions are similar or greater than the mean current the hydrodynamic roughness z0 determined from velocity profiles is substantially larger than directly measured values the roughness signature in turbulent energy spectra persists with elevation when its scale falls in the dissipation range but decays in the log layer for larger roughness elements reynolds shear stress profiles peak in the lower parts of the log layer diminishing below it and gradually decaying at higher elevations in contrast wave shear stresses are negligible within the log layer but become significant within the roughness sublayer this phenomenon is caused by an increase in the magnitude and phase lag of the vertical component of wave induced motion no single boundary layer length scale collapses the reynolds stresses but both the prandtl mixing length and eddy viscosity profiles agree well with the classical model of linear increase with elevation especially near the seabed within the log region profiles of shear production and dissipation rates of turbulence converge below it dissipation rapidly increases peaking near the seabed conversely the shear production decays near the seabed in agreement with the eddy viscosity model but in contrast to both laboratory and computational rough wall studies

palynological evidence of climatic and oceanographic changes in the north sea during the last deglaciation abstract palynological analyses performed on cores from the norwegian channel troll 8903 led to reconstruction of the late glacial variations in sea surface conditions using dinoflagellate cyst data and permitted direct correlation with the vegetation history of northwestern europe derived from pollen assemblages by 15 000 yr b p ice rapidly receded from the norwegian shelf and relatively warm summer conditions prevailed in surface waters a first late glacial cooling marked by extensive seasonal sea ice cover is dated at ca 13 600 13 00014c yr b p which coincides with the oldest dryas interval during the bolling allerod interval a rise in sea surface temperature both in february up to 3 c and august up to 15 c led to the establishment of ice free conditions in the northern north sea while pollen data reveal a densification of the vegetation cover the beginning of the younger dryas interval is marked by an increase in nonarboreal pollen input indicative of the opening of the forest vegetation cover concomitant with a cooling of surface waters during winter and development of sea ice cover however sea surface conditions remained relatively warm in summer until about 10 300 yr b p when extremely cold conditions and extensive sea ice cover developed up to 7 months yr improving conditions are recorded in surface waters by 10 100 yr b p a few hundred years before the development of forest cover onshore as shown by the pollen record such a discrepancy between marine and terrestrial indicators at the end of younger dryas time suggests a delayed response of the vegetation to regional climate warming

extraterrestrial impact events the record in the rocks and the stratigraphic column abstract the known terrestrial impact record is a biased sample of a much larger population of impact events the biases are due to the modifying effects of terrestrial geologic processes coupled with incomplete searches for impact structures and impact related materials terrestrial impact structures have the same basic forms as impact craters on the other planets of the inner solar system but because of post impact modification by terrestrial geologic process are recognised by the occurrence of shock metamorphic effects in some cases siderophile anomalies have been identified in impact lithologies and have been used to estimate the composition of the impacting body similar shock metamorphic effects and a siderophile anomaly in k t boundary materials are indicative of a major impact event which has been correlated with the formation of the chicxulub structure mexico evidence of a small number of other impacts occur in the stratigraphic record most commonly as tektite or microtektite horizons in some cases they are known to be accompanied by geochemical anomalies in other cases a number of ir anomalies have been reported in the stratigraphic record but there is no confirmatory evidence that they are due to impact the majority of known impact events in the stratigraphic record are from relatively recent geologic time logic dictates however that many more impacts must be recorded in terrestrial sediments and model calculations indicate that relatively small impacts d 20 km have the potential to cause atmospheric blow out and thus global dispersion of some of the impact products geochemical detection however of such events may not be easy in some cases because of relatively small absolute signals against the background of the daily infall of cosmic material in addition non chondritic bodies may result in no appreciable geochemical anomaly in view of this any claim to a geochemical signature of impact in the stratigraphic record should be accompanied by a physical search for impact materials although in the case of impacts into oceanic crust this too will be difficult given the k t experience however and the fact that large scale impact on earth is a natural consequence of the character of the solar system the potential of impacts to provide local and global marker horizons can not be ignored similarly the fact that impacts may have the potential to result in short term biologic or climatic excursions can not be dismissed arbitrarily when considering the causes of such phenomena as stable isotope anomalies in the stratigraphic record

the use of radiocarbon derived δ13c as a paleoclimate indicator applications in the lower alentejo of portugal abstract values of 13 c are frequently reported with radiocarbon dates from organic materials in c 3 plants 13 c values have been linked to changes in water use efficiency as a response to arid conditions by calculating 13 c discrimination 13 c from 13 c isotopic composition 13 c archaeologists can gain potentially valuable inference into past climate conditions values of 13 c reflect the process of discrimination against heavier 13 c isotopes of carbon by comparing the 13 c of samples to that of the atmosphere and can be calculated when records of atmospheric 13 co 2 are available the present study examines a 1300 year history of radiocarbon derived 13 c from the lower alentejo of portugal using charcoal recovered from excavations of a series of medieval habitation sites in the study area to calculate 13 c the posterior means generated from bayesian change point analysis of 13 co 2 records were used archaeological data were then compared to contemporary ecological studies of 13 c of the same taxa against instrumental records of climate values of 13 c fell within mean ranges for the taxa through a period of population growth between the 7th and 10th centuries ad during the height of the medieval warm period in the 11th century ad 13 c values frequently fell to low levels associated with arid conditions at this time environmental degradation and erosion were documented values of 13 c increased for a brief period in the early 12th century ad before the rural lower alentejo was largely abandoned for nearly two centuries another period of aridity occurred in the 16th and 17th centuries ad radiocarbon derived 13 c is a potentially useful paleoclimate proxy for archaeologists provided that results can be paired with observed 13 c variation in studies that pair these data with instrumental climate records

holocene climatic change in the northern midwest pollen derived estimates mapping of holocene pollen data in the midwestern united states has revealed several broadscale vegetational changes that can be interpreted in climatic terms these changes include 1 the early holocene northward movement of the spruce dominated forest and its later southward movement after 3000 yr b p and 2 the eastward movement of the prairie forest border into southwestern wisconsin by 8000 yr b p and its subsequent westward retreat after 6000 yr b p when certain basic assumptions are met multiple regression models can be derived from modern pollen and climate data and used to transform the pollen record of these vegetational changes into quantitative estimates of temperature or precipitation to maximize the reliability of the regression equations we followed a sequence of procedures that minimize violations of the assumptions that underlie regression analysis reconstructions of precipitation during the holocene indicated that from 9000 to 6000 yr b p precipitation decreased by 10 to 25 over much of the midwest while mean july temperature increased by 0 5 to 2 0 c at 6000 yr b p precipitation was less than 80 of its modern values over parts of wisconsin and minnesota after 6000 yr b p precipitation generally increased while mean july temperature decreased in the north and increased in the south the time of the maximum temperature varies within the midwest and is earlier in the north and later in the south

model data comparisons of moments of nonbreaking shoaling surface gravity waves the predictions of linear and nonlinear boussinesq shoaling wave models for nonbreaking unidirectional surface gravity waves are compared to field observations with particular emphasis on quantities that may be important for cross shore sediment transport the extensive data sets were obtained on two natural beaches span water depths between 1 and 10 m and include incident wave power spectra with narrow broad and bimodal shapes significant wave heights varied between approximately 30 and 100 cm and peak periods between approximately 8 and 18 s the evolution of total variances of sea surface elevation cross shore velocity and horizontal acceleration is modeled at least qualitatively well by both linear and nonlinear theories only the nonlinear theory predicts the increasingly asymmetric sea surface elevations and horizontal velocities pitched forward wave shapes and the weaker variation of skewness difference between crest and trough profiles which are observed to occur during shoaling the nonlinear theory also models qualitatively well the large skewed accelerations which occur during the passage of asymmetric waves

extraterrestrial influences on mantle plume activity abstract we use time series analysis to compare the impact histories of the earth and moon with the record of mantle plume activity we use events with errors in their ages of 150 ma the terrestrial and lunar impact records when smoothed at a 45 ma interval correlate at a 97 confidence level this high confidence level suggests that we have an adequate sampling of most of the major impact events on the earth we then test the idea that existing mantle plumes may be strengthened by impacts when smoothed at a 45 ma interval strong plumes correlate with the terrestrial impact record at better than a 99 confidence level no time lag is discernible between the data sets which is expected given their present error level when the time series are smoothed at a 30 ma interval there are 10 major peaks in impact activity nine out of ten of these peaks have a counterpart in either or both of the strong mantle plume or the mantle plume time series as a result the strong mantle plume and the impact time series correlate at the 97 confidence level the mantle plume and the impact time series correlate at the 90 confidence level finally the deccan plume showed greatly increased activity immediately after the chixculub impact the results of our analysis suggest that large meteorite and cometary impacts may well increase the amount of volcanism from already active mantle plumes

bed slope effects on turbulent wave boundary layers 1 model validation and quantification of rough turbulent results 1 a numerical model solving incompressible reynolds averaged navier stokes equations combined with a two equation k turbulence closure is used to study converging diverging effects from a sloping bed on turbulent oscillatory wave boundary layers bed shear stresses from the numerical model are first validated against hydraulically smooth and rough data from flat bed experiments in the form of wave friction factor diagrams the results show that the model provides acceptable accuracy for wave orbital amplitude to roughness length ratios a kn 20 validation against experimental measurements for steady streaming induced by a skewed free stream velocity signal is also provided we then simulate a series of experiments involving oscillatory flow in a convergent divergent smooth tunnel and a good match with respect to bed shear stresses and streaming velocities is achieved the streaming is conceptually explained using analogies from steady converging and diffuser flows a parametric study is undertaken to assess both the peak and time averaged bed shear stresses in converging and diverging half periods under rough turbulent conditions the results are presented as friction factor diagrams a local similarity condition is derived for relating oscillatory flow in a convergent divergent tunnel as considered herein to shoaling shallow water waves by matching spatial gradients in the free stream orbital length

late devensian and holocene palaeoecology and palaeoenvironments of the morrone birkwoods aberdeenshire scotland results are presented from analyses of pollen vascular plant macrofossils and bryophyte remains at the morrone birkwoods a site in the eastern highlands of scotland the sedimentary record spans the past ca 12 500 yr and is dated by a series of radiocarbon determinations quantitative palaeoclimate reconstructions are made using pollen climate response surfaces with the selection of analogues constrained using evidence from the macrofossil stratigraphy the results generally are in remarkably close agreement with those reconstructed from beetle remains by atkinson et al the only significant discrepancy is with respect to the extent of the warming at ca 12500 yr bp the palaeoclimate record indicates a marked cooling that coincides in age with the older dryas event and which interrupts an overall cooling trend throughout the duration of the late glacial interstadial that culminates in the younger dryas there is also evidence of a climate fluctuation during the first 500 yr of the holocene the mid and late holocene record of palaeoclimate is in contrast remarkably invariant until the last ca 1500 yr during which there is evidence of a period of warming followed by a subsequent cooling it is speculated that these may correspond to the medieval warm period and to the little ice age respectively the morrone birkwoods are noted today for their sub arctic vegetation and their rich flora with the arctic alpine and northern montane biogeographical elements especially well represented the macrofossil record indicates that the flora has long had this general character whilst both this and the pollen record indicate that the sub arctic character of the vegetation has most likely persisted since earliest holocene times

aridity of the mediterranean sea at the last glacial maximum a reinterpretation of the δ18o record paleoclimate data show conflicting evidence for aridity in the mediterranean basin at the last glacial maximum lgm two recent modeling studies of the lgm mediterranean using water balance biome and ocean general circulation models respectively suggest that evaporation over the basin was reduced at the lgm compared with today in apparent conflict with these results is the strongly enhanced 18o signature of the lgm mediterranean surface waters particularly in the eastern basin which has been ascribed to a much higher salinity in an alternative interpretation presented here a box model of the balance between paleoclimate model evaporation and rainfall and paleoceanographic model advective terms shows that the observed lgm 18o signature can be reproduced without the need for an enhanced salinity with advection playing the dominant role in the 18o budget the box model also shows that the 18o data alone are insufficient to determine the type of exchange between the eastern and western basins the results emphasize that regional paleoceanographic water balances should consider all relevant processes and not just the evaporative flux

on the average shape of the largest waves in finite water depths abstractthis paper investigates the average shape of the largest waves arising in finite water depths specifically the largest waves recorded in time histories of the water surface elevation at a

vertical distribution of wave shear stress in variable water depth theory and field observations 1 a generalized analytical model is developed to describe the vertical distribution of wave induced shear stress in the presence of bottom slope bottom friction and depth induced wave breaking in the shoaling region and surf zone the theory is compared to data obtained on an unbarred beach in 3 7 m mean water depth over a 2 bottom slope bottom slope and bottom friction are incorporated in the theory by including the wave energy dissipation by bottom friction in a potential wave theory for variable water depth and coupling the wave theory with a wave bottom boundary layer wbbl theory over a sloping bottom the effect of wave breaking is included through a periodic bore dissipation model previous studies have considered some but not all of these effects each of the three components sloping bottom bottom friction and wave breaking exhibits a different vertical structure the contributions due to bottom slope and bottom friction attain maximum strength in the wbbl and decay with distance above the bed approaching smaller but nonzero values at the surface in contrast the contribution from wave breaking is maximal at the surface and decays linearly with depth becoming zero at the bed the vertical structure of the cross shore u and vertical w components of wave orbital velocity was measured using a coherent doppler acoustic profiler the measured velocity fields were used to obtain the ensemble averaged wave shear stress profiles that extend across the wbbl to a height of 30 cm above the bed close to the seabed the observed and predicted uw profiles are similar confirming the sensitivity of wave stress to frictional and bottom slope effects within the wbbl farther from the bed however the observed profiles fall off more rapidly with height as w approaches quadrature with u faster than predicted wave breaking induced shear stresses were not observed

reconstruction of the mid holocene palaeoclimate of siberia using a bioclimatic vegetation model abstract a bioclimatic vegetation model is used to reconstruct the palaeoclimate of siberia during the mid holocene a warm moist period also known as the holocene climatic optimum our goal is to determine the magnitude of climatic anomalies associated with mapped changes in vegetation classes reconstructed anomalies are the logical outcome of the bioclimatic assumptions in the siberia vegetation model operating on location specific differences in the palaeomap of khotinsky and the modern map of isachenko the siberian vegetation model specifies the relationship between vegetation classes and climate using climatic indices growing degree days dryness index continentality index these indices are then converted into parameters commonly used in climatic reconstructions january and july mean temperatures and annual precipitation climatic anomalies since the mid holocene are then displayed by latitude and longitude an advantage of a model based approach to climatic reconstruction is that grid cells can be modelled independently without the need for interpolation to create smoothed temperature and precipitation contours the resulting pattern of anomalies is complex on average siberian winters in the mid holocene were 3 7 c warmer than now with greater warming in higher latitudes the major winter warming was concentrated in the taiga zone on the plains and tablelands of east siberia where a warm and moist climate was necessary to support a broad expanse of shade tolerant dark needled taiga january temperatures averaged about 1 c warmer than now across southern siberia although large areas show no change july temperature anomalies 0 5 c are distributed mostly latitudinally with anomalies increasing with latitude above 65 n at latitudes below 65 n july temperature was nearly the same as today across siberia based on july temperatures siberian summers in the mid holocene were 0 7 c warmer than today s annual precipitation in siberia was predicted to be 95 mm greater in the mid holocene than now most of the increase was concentrated in east siberia 154 mm average increase the precipitation anomalies are small in the south large precipitation anomalies are found in central and northeastern siberia this location corresponds rather closely to the large anomalies in january temperature in east siberia the annual precipitation increase was 200 mm more than present precipitation in yakutia this increase corresponds to the deep penetration of moisture demanding dark needled species pinus sibirica abies sibirica picea obovata into east siberia in the mid holocene where currently only drought resistant light needled species larix spp are found another area of increased precipitation was along the polar circle in west siberia and at the base of the taymyr peninsula in east siberia in combination with 2 5 c warmer summers moister climates there allowed forests to advance far northward into what is now the tundra zone

skewness in the nearshore zone a comparison of estimates from marsh mcbirney current meters and colocated pressure sensors many of the field studies that have undertaken an investigation of surf zone dynamics e g nearshore sediment transport study canadian coastal sediment study have relied on marsh mcbirney electromagnetic current meters to measure the velocity field data from these instruments have been used to compute higher order moments of the velocity field which are used as an input in many sediment transport models however recent laboratory measurements by aubrey and trowbridge 1985 have raised questions regarding the suitability of marsh mcbirney current meters for surf zone studies and the accuracy of higher order velocity moments fortunately colocated pressure and velocity measurements were made during field programs in effect this provides a calibration check in the field of the performance of these instruments a cross spectral analysis indicates that colocated marsh mcbirney current meter measurements are highly coherent this is true for measurements from inside and outside the surf zone as a result normalized skewness estimates computed with the aid of the bispectrum from these colocated current meter measurements differ by only a few percent a cross spectral analysis between colocated pressure and velocity measurements show a similar distribution of variance for wind wave frequencies observed differences between normalized skewness estimates from colocated pressure and velocity measurements are shown to be consistent with the theoretical expectation that pressure has a nonlinear dependence on velocity these results suggest that the marsh mcbirney current meter is suitable for the estimation of nondimensional velocity moments even in breaking wave conditions

reconstruction of climate and vegetation changes of lake bayanchagan inner mongolia holocene variability of the east asian monsoon a high resolution pollen and pediastrum record spanning 12 500 yr is presented for lake bayanchagan 115 21 e 41 65 n and 1355 m a s l southern inner mongolia individual pollen taxa pt mat and the pft affinity scores pft mat were used for quantitative climatic reconstruction from pollen and algal data both techniques indicate that a cold and dry climate similar to that of today prevailed before 10 500 cal yr b p the wettest climate occurred between 10 500 and 6500 cal yr b p at which time annual precipitation was up to 30 60 higher than today the early holocene increases in temperature and precipitation occurred simultaneously but mid holocene cooling started at approximately 8000 cal yr b p 1500 yr earlier than the drying vegetation reconstruction was based on the objective assignment of pollen taxa to the plant functional type the results suggest that this region was dominated by steppe vegetation throughout the holocene except for the period 9200 to 6700 cal yr b p when forest patches were relatively common inner mongolia is situated at the limit of the present east asian monsoon and patterns of vegetation and climate changes in that region during the holocene probably reflect fluctuations in the monsoon s response to solar insolation variations the early to middle holocene monsoon undoubtedly extended to more northern latitudes than at present

a palaeoecological approach to the last 21 000 years in the pyrenees the peat bog of freychinede alt 1350 m ariege south france local palaeoclimatic and palaeoecological reconstructions in the ariege pyrenees were applied to palynological and sedimentological investigations of the freychinede peat bog alt 1350 m complemented by a geomorphological study of the neighbouring area correlations are possible between local glacial evolution deposition in the lake and the history of the vegetational cover for the period from about 21 300 b p to 13 000 b p the altitude of the cirque floors indicates mean july temperatures probably ranging between 6 5 c and 8 5 c from 21 300 b p to 15 000 b p 7 c and 9 c from 15 000 b p to 13 150 b p 7 5 c and 9 5 c from 13 150 b p to the beginning of the final transitional phase based on the comparison of the palynological data with the modern plant communities of the area especially with the subalpine and alpine communities the local climate would have been favourable to the development of boreo arctic plants at the altitude of the lake during the cold phase the postglacial period begins near 10 650 b p with the extension of quercus abies spread at about 7000 b p followed by tilia cordata from 5000 to 3900 b p fagus spread moderately but its strongest development occurred after about 3900 3800 b p when human activity began on the mountain forest and many beech fir forests developed middle ages in many valleys of ariege the mountain and subalpine forests were destroyed the modern landscape has large treeless zones with scattered recent populations of beech where fir is beginning to develop

reynolds stresses under wind waves this paper describes a series of wave measurements taken from an oceanographic tower concerning water particle kinematics attention is focused on the phase relationship between surface elevation and the velocity components large phase shifts with respect to the expected values are found under active wave conditions while swell behavior closely follows the linear theory the reliability of the measurements is analyzed in detail possible physical reasons for the findings are indicated in the final discussion

a wavelet based algorithm to estimate ocean wave group parameters from radar images in recent years new remote sensing techniques have been developed to measure two dimensional 2 d sea surface elevation fields the availability of these data has led to the necessity to extend the classical analysis methods for one dimensional 1 d buoy time series to two dimensions this paper is concerned with the derivation of group parameters from 2 d sea surface elevation fields using a wavelet based technique wave grouping is known to be an important factor in ship and offshore safety as it plays a role in dangerous resonance phenomenons and the generation of extreme waves synthetic aperture radar sar data are used for the analysis the wavelet technique is introduced using synthetic ocean surfaces and simulated sar data it is shown that the group structure of the ocean wave field can be recovered from the sar image if the nonlinear imaging effects are moderate the method is applied to a global dataset of european remote sensing satellite ers 2 wave mode data different group parameters including the area covered by the largest group and the number of groups in a given area are calculated for over 33 000 sar images global maps of the parameters are presented for comparison classical 1 d grouping parameters are calculated from colocated wave model data showing good overall agreement with the wavelet derived parameters ers 2 image mode data are used to study wave fields in coastal areas waves approaching the island of sylt in the north sea are investigated showing the potential of the wavelet technique to analyze the spatial wave dynamics associated with the bottom topography observations concerning changes of wavelength and group parameters are compared to linear wave theory

the climate in western europe during the last glacial interglacial cycle derived from pollen and insect remains using the pollen sequence of la grande pile xx france we review problems with the application of transfer functions in paleoclimatic reconstructions one of them is to find modern analogues for the herbaceous vegetation of the cold periods we propose a method to distinguish between steppes and tundra vegetations for which the moderns are only partial analogues of the glacial periods another method to solve these problems is based on constraining by insect remains the two methods provide coherent reconstructions the results are also compared with other paleodata there is a good correlation with the six cold heinrich events between 70 and 15 ka b p a cooling event during the eemian period marked by high percentages of taxas at about 125 ka b p needs still to be correlated with high resolution ocean and ice cores

characterization of turbulent airflow over evolving water waves in a wind wave tank 1 results of an experimental investigation of the turbulent boundary layer in airflow over evolving young wind waves are presented the experiments were conducted in a laboratory wind wave flume consisting of a closed loop wind tunnel capable of generating wind speed that may exceed 15 m s atop of a 5 m long wave tank simultaneous measurements of mean wind velocity and of instantaneous fluctuations of the horizontal and vertical air velocity components were carried out along the test section at different airflow rates and at numerous heights above the highest wave instantaneous surface elevation at the air sensors location was simultaneously recorded the friction velocities at all locations and for all airflow rates were determined by two independent methods by fitting the logarithmic velocity profiles and by extrapolating the measured reynolds shear stresses to mean water surface level the variation with height and along the test section of the fluctuations of two velocity components in the mean flow and in the vertical directions was also studied and the results compared with flow behavior over rough and smooth plates wave induced airflow parameters were then investigated by application of cross spectral analysis results on the vertical extent of wave induced boundary layer on the phase relation between the wave induced velocity fluctuations and the surface elevation as well as on the wave induced reynolds shear stress are reported

rates of change and chronological problems during the late glacial period results of high resolution ams 14c dating of terrestrial plant macrofossils from late glacial and early holocene lake deposits in switzerland show three periods with constant radiocarbon ages these plateaux of constant age occur at 12700 10000 and 9500 y bp a comparison of this radiocarbon chronology with a varve chronology documents discrepancies between the sidereal and the radiocarbon time scale for the late glacial period the age plateaux and the time scale discrepancies have a significant impact on the estimation of rates of change during this period estimates of rates of change can be very misleading if calculated on the basis of radiocarbon ages this is illustrated by an example of estimated rates of late glacial and early holocene palynological change in switzerland

residence time of chalk groundwaters in the paris basin and the north german basin a geochemical approach the comparative geochemical and isotopic study of confined and unconfined chalk groundwaters of the paris basin and the n german basin proves a significant chemical evolution during groundwater flow from the recharge zones to the deep confined aquifer different time dependent geochemical parameters have been tested as dating tools cation ratios sr2 ca2 mg2 ca2 n no 3 noble gas contents as paleotemperature indicators ne ar kr xe radiogenic he 13c 14c 18o 2h 3h cation ratios and 13c show the importance of incongruent dissolution processes in the chalk aquifer water rock interactions were taken into account in a multi step dissolution model to determine radiocarbon groundwater ages the oldest waters in the confined part of the paris basin chalk with maximum 14c ages of 14 000 a b p contain pleistocene recharge components as can be shown by a stable isotope depletion and noble gas temperatures significantly lower than in recent groundwaters chalk waters at the lagerdorf site in northern germany show a distinct stratification with respect to residence times and hydrochemistry

the dynamic pressure in deep water extreme stokes waves in this paper we consider the dynamic pressure in a deep water extreme stokes wave while the presence of stagnation points introduces a number of mathematical complications maximum principles are applied to analyse the dynamic pressure in the fluid body by means of an excision process it is shown that the dynamic pressure attains its maximum value beneath the wave crest and its minimum beneath the wave trough while it decreases in moving away from the crest line along any streamline this article is part of the theme issue nonlinear water waves

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

vegetation and climate of the last interglacial on baffin island arctic canada abstract sediment cores recovered from three mid arctic lakes on cumberland peninsula eastern baffin island arctic canada contained two units of unconformably superimposed organic sediment gyttja the upper of which represented sedimentation during the holocene radiocarbon ages on aquatic macrofossils from the lower gyttja suggested it to be 50 ka bp while luminescence ages from two cores constrained the age of this sediment to 90 and betula and alnus pollen grains than did overlying holocene sediments numerical comparisons of fossil pollen assemblages to a data set of 400 modern high latitude lake sediment samples revealed that the last interglacial vegetation of east central baffin island was low arctic in character comparable to present day southwest greenland from applications of both correspondence analysis regression and best modern analogue methodologies we infer july air temperatures of the last interglacial to have been 4 to 5 c warmer than present on eastern baffin island which was warmer than any interval within the holocene on these grounds we ascribe the lower lacustrine unit in these lakes to the climatic optimum of the last interglacial ca 117 to 130 ka bp marine isotopic stage 5e

quantitative interpretation of fossil pollen spectra dissimilarity coefficients and the method of modern analogs abstract dissimilarity coefficients measure the difference between multivariate samples and provide a quantitative aid to the identification of modern analogs for fossil pollen samples how eight coefficients responded to differences among modern pollen samples from eastern north america was tested these coefficients represent three different classes 1 unweighted coefficients that are most strongly influenced by large valued pollen types 2 equal weight coefficients that weight all pollen types equally but can be too sensitive to variations among rare types and 3 signal0to noise coefficients that are intermediate in their weighting of pollen types the studies with modern pollen allowed definition of critical values for each coefficient which when not exceeded indicate that two pollen samples originate from the same vegetation region dissimilarity coefficients were used to compare modern and fossil pollen samples and modern samples so similar to fossil samples were found that most of three late quaternary pollen diagrams could be reconstructed by substituting modern samples for fossil samples when the coefficients indicated that the fossil spectra had no modern analogs then the reconstructed diagrams did not match all aspects of the originals no modern analogs existed for samples from before 9300 yr b p at kirchner marsh minnesota and from before 11 000 yr b p at wintergreen lake michigan but modern analogs existed for almost all holocene samples from these two sites and brandreth bog new york

comet showers as a cause of mass extinctions if at least some mass extinctions are caused by impacts why do they extend over intervals of one to three million years and have a partly stepwise character the solution may be provided by multiple cometary impacts astronomical geological and palaeontological evidence is consistent with a causal connection between comet showers clusters of impact events and stepwise mass extinctions but it is too early to tell how pervasive this relationship may be

effects of sea maturity on satellite altimeter measurements for equilibrium arid near equilibrium sea states the wave slope variance is a function of wind speed u and of the sea maturity the influence of both factors on the altimeter measurements of wind speed wave height and radar cross section 0 is studied experimentally the necessary theoretical background both on wave statistics and on the near nadir radar backscatter is outlined it is based on a recently proposed view that the effective fractal dimension for a surface patch corresponding to the equilibrium range of wave spectra increases as the wave field matures the experimental part is based on 1 year s worth of geosat altimeter observations colocated with in situ wind and wave measurements by 20 noaa buoys errors and biases in altimeter wind speed and wave height measurements are investigated a geophysically significant error trend correlated with the sea maturity i e with c0 u where c0 is the phase velocity of waves corresponding to the spectral peak and u is the mean wind speed well above the surface is found in wind speed measurements this trend is explained by examining the effect of the generalized wind fetch x gh1 32 u2 on the curves of the observed dependence 0 f u x we conclude in particular that unambiguous measurements of wind speed by altimeter in a wide range of sea states are impossible without accounting for the actual degree of wave development an approach to a more accurate retrieval of wind speeds is suggested as based on accounting for the actual degree of wave development which can be achieved through processing the total radar cross section simultaneously with the significant wave height information contained in altimeter waveforms the potential capability of a satellite altimeter to estimate wave age and similar measures of wind wave coupling is discussed this capability expands the use of satellite altimeters for air sea interaction studies

pollen based summer temperature reconstructions for the eastern canadian boreal forest subarctic and arctic pollen based paleoclimate reconstructions using response surface and modern analog methods reveal an 8000 year record of july temperature fluctuations for 25 eastern canadian lake sites located from the forest tundra to the high canadian arctic postglacial conditions characterized by warmer than present summer temperatures prevailed in baffin island and ne labrador beginning about 7500 and 7000 14c yr bp respectively resulting in warmer than present conditions throughout the region by 6000 14c yr bp 0 5 c to 1 c further south in quebec and w labrador july temperatures were 1 2 c colder than present until after 6000 14c yr bp and only reached modern values after all residual laurentide ice had melted increased summertime insolation and the final disappearance of laurentide ice during the middle holocene probably caused july temperatures throughout eastern canada to peak between approximately 5000 and 3500 14c yr bp mid holocene warming relative to today was more pronounced in baffin island and ne labrador 1 c to 2 c compared to the boreal and subarctic regions of quebec and w labrador 1 c over the past 4000 years decreasing summertime insolation and colder sea surface temperatures in the davis strait and labrador sea contributed to a decline in july temperatures of 1 2 c throughout baffin island and the tundra regions of n quebec and labrador the absence of similar cooling in the records from the boreal forest may support the notion that the mean position of the summertime polar front blocked the colder arctic air during the late holocene

dissimilarity mapping between fossil and contemporary pollen spectra in europe for the past 13 000 years abstract the degree of analogy between fossil and contemporary pollen spectra in europe has been investigated using the chord distance dissimilarity measure no analog pollen spectra represent vegetation without a modern analog and hence by inference represent macroclimatic conditions different from any occurring in the region today such spectra have minimum chord distances that exceed a threshold value assessed using contemporary samples from the same and different vegetation u units contoured maps of minimum chord distance portray the changing patterns of analogous and no analog pollen spectra and hence vegetation units since 13 000 yr b p no analog vegetation units have been extensive in some regions for much of the holocene persisting as recently as 1000 years ago in many areas the chord distance measure has also been used to explore the patterns extent and rates of change in european pollen spectra since 13 000 yr b p pollen spectra changed rapidly during late glacial and early holocene times and during the last millennium paleoclimatic changes have brought about the major changes in the holocene paleovegetation of europe human impact upon european vegetation has obscured neither the contemporary relationship between pollen spectra and vegetation nor the climatically determined long term changes of vegetation across the continent since 13 000 yr b p

the oceanographic tower acqua alta activity and prediction of sea states at venice abstract the oceanographic tower acqua alta was constructed in early 1970 in the adriatic sea some 20 km from venice the main objective was to obtain information about the meteomarine conditions off the venice lagoon where the old town is threatened by ever more severe floodings the paper describes the tower and its instrumentation and summarizes the experience and results of activities relevant to coastal engineering this encompasses modelling of wind field storm surge and wind generated waves various types of models have been applied to and validated for the adriatic sea and in particular for the coastline of the venice lagoon this includes application of long term statistics of extreme wave heights specific studies of the influence of bottom friction and wind gustiness on waves are presented as well as measurements of kinematics of wind generated waves further a short discussion of white capping is included finally some information about the management of the tower is given

late quaternary climatic change in france estimated from multivariate pollen time series abstract in regions like southern france where usual analytical techniques are unsatisfactory due to the heavy influence of human activity and the existence of a complex climate the quantitative reconstruction of climate from pollen is particularly difficult that is why an original method has been performed the first step of the method based on best analogs estimation and multiple regression is to calculate a relationship between climate monthly temperature and precipitation and modern pollen spectra for data from 182 sites the result is called the analog climate the second step of the method an original combination of canonical correlation and principal components analyses extracts the common information from several fossil pollen sequences to produce a so called paleobioclimate this step removes the effects of local differences in the vegetation among nearby sites and also reduces the effects of human disturbance in the third step the signals obtained from the first two steps are merged by kalman filter into a final reconstruction where the noise is reduced by around 37 the results presented with a 95 confidence level suggest that the pleni wurm was 7 to 13 c colder than present and had 20 to 60 less precipitation than today this large confidence takes into account the climatic diversity of the region at 13 000 yr b p the climate reached the modern level of precipitation but the temperature was just less than present the subsequent decrease of precipitation is less important than that of temperature this fact may explain the advance of alpine glaciers during the younger dryas preboreal warming was abrupt 3 to 4 c per 500 yr and precipitation increased more slowly

on estimating extremes in an evolving wave field abstract observations of waves as a time series from a fixed or moored sensor are shown to underestimate the extreme waves occurring in the vicinity the underestimate arises because of the difference in phase and envelope propagation of surface water waves so that the highest crest for example is recorded only when the crest coincides with the envelope maximum at the location of the wave recorder in addition the dispersive nature of water waves can lead to the coalescence of groups of waves as longer waves catch up to shorter slower waves so that the group shape changes markedly as the waves propagate moored accelerometer buoys introduce another error that exacerbates the underestimate of the highest crest the quasi lagrangian motion of the buoy leads to flattening of the crests and sharpening of the troughs so that apparent mean water level is raised and thus the height of crests above it is reduced an analysis approach to correct these underestimates is outlined and tested with observed data against the predictions of extreme values based on narrow banded random wave theory

a perspective on the evidence for periodic cometary impacts on earth abstract recent suggestions of periodic cometary showers coincident with mass extinctions have prompted a review of the terrestrial cratering record from the point of view of completeness of search and crater retention the record used to argue for statistical periodicities is incomplete and may not be representative estimates of crater ages are of variable accuracy and precision and in some cases different dating methods yield discordant results it is possible to define through time series analysis a number of periodicities of different magnitude 29 m y 21 m y 18 5 m y 13 5 m y and phase depending on which craters are considered the most representative sample of the record arguing against periodic cometary showers is the observation that estimates of the average cratering rate in recent time and observations on apollo bodies are equivalent in addition siderophile element data from impact melt rocks suggest a variely of projectile types were responsible for relatively large terrestrial craters given these observations it is concluded that statements regarding periodic cometary showers are not categorical and require evidence beyond arguments based on time series analyses of an incomplete cratering record

climate variability in the aral sea basin central asia during the late holocene based on vegetation changes high resolution pollen analyses 50 yr from sediment cores retrieved at chernyshov bay in the nw large aral sea record shifts in vegetational development from subdesertic to steppe vegetation in the aral sea basin during the late holocene using pollen data to quantify climatic parameters we reconstruct and date for the first time significant changes in moisture conditions in central asia during the past 2000 yr cold and arid conditions prevailed between ca ad 0 and 400 ad 900 and 1150 and ad 1500 and 1650 with the extension of xeric vegetation dominated by steppe elements these intervals are characterized by low winter and summer mean temperatures and low mean annual precipitation pmm 250 mm yr conversely the most suitable climate conditions occurred between ca ad 400 and 900 and ad 1150 and 1450 when steppe vegetation was enriched by plants requiring moister conditions pmm 250 500 mm yr and some trees developed our results are fairly consistent with other late holocene records from the eastern mediterranean region and the middle east showing that regional rainfall in central asia is predominantly controlled by the eastern mediterranean cyclonic system when the north atlantic oscillation nao is in a negative phase

the velocity field beneath wind waves observations and inferences an extensive set of measurements taken from a fixed tower is used to study the velocity field under wind waves velocity measurements made with miniature drag spheres are compared with linear theory estimates of the orbital velocities obtained from measured surface elevation results are presented in the context of how well linear theory is able to predict wave induced forces on cylindrical structural members linear theory is seen to predict the flow velocities to within about 7 furthermore both the inertial and drag forces are generally well predicted by linear theory although small scale turbulent motions not accounted for by linear theory can result in significantly higher inertial forces on smaller structural members

periodicity of extinctions in the geologic past deterministic versus stochastic explanations the temporal spacing and the magnitude of major extinctions over the past 250 and 570 million years based on the use of different metrics of extinction probability are analyzed by comparing deterministic and stochastic explanations the best fitting time series model is a stochastic autoregressive model that displays a pseudoperiodic behavior with a cycle length of 31 million years for the past 250 million years regardless of the metric of extinction probability the periodicity lengthens and weakens when the analysis is extended to the entire phanerozoic the history of the probability of extinction for the entire phanerozoic based on time series analysis does not support the reported bipartite distribution of van valen rather the probability of extinction has decreased uniformly over phanerozoic time whereas the inertia or stability of the biotic system after the late permian crisis has increased

reconstruction of holocene precipitation patterns in europe using pollen and lake level data abstract lake level data can be used to refine palaeoclimate reconstructions based on pollen data this approach is illustrated for the european holocene estimates of p pet precipitation minus potential evapotranspiration were first inferred from modern pollen analogues the pollen based estimates were then compared with the status of lakes within a 5 radius analogues with p pet anomalies inconsistent with the lake level changes were rejected the constrained sets of analogues were used to estimate continental scale patterns of annual mean temperature and annual precipitation at 3000 yr intervals estimated temperature anomalies differed only slightly from the unconstrained reconstructions estimated precipitation anomalies however showed improved spatial coherence and increased regional contrast and were occasionally reversed in sign the effect of the constraint was to impose a rational selection among almost equally similar modern pollen analogues with similar temperatures but widely varying moisture regimes the resulting maps showed clear spatially coherent patterns of change in precipitation as well as temperature suitable for comparison with climate model results further improvement of these maps will become possible as a more extensive coverage of lake level data is obtained

the weichselian late glacial in southwestern europe iberian peninsula pyrenees massif central northern apennines this paper presents a summary of late glacial environmental changes in southwestern europe lberian peninsula pyrenees massif central and the northern apennines the emphasis is on palaeoclimatic interpretations inferred from key sites in the region from which the most detailed records are available and which have been radiocarbon dated the earliest evidence for climatic improvement following the end of the last glacial stage is dated to ca 15 ka bp and is found at a few sites only by 13 ka bp a more widespread and marked climatic improvement is evident although it is difficult to be precise in the timing and magnitude of the event there are significant variations in detail between the late glacial records but evidence for a significant cooling correlated with the younger dryas event is widespread throughout the region just two sites in the region provide evidence for an earlier less emphatic phase of climatic cooling which is tentatively equated with the older dryas of continental northern europe dry conditions appear to have predominated throughout the region in the later part of the younger dryas and the early holocene

observations of wave shear stress on a steep beach observations are presented of the wave shear stress u w on a steeply sloping beach above the wave boundary layer wbl positive values of u w were observed and are attributed to a combination of both wave shoaling due to the large scale bed slope and dissipation due to wave breaking in agreement with the wave theory of zou et al 2003 within the wbl observed vertical profiles of u w were also in good agreement with theory in cases where the wave height was small as wave heights increased however the wbl profile of u w generally did not agree with theory near simultaneous rotary sonar observations of the bed suggest the disagreement with theory was due to the presence of orbital scale ripples which the present theory does not accommodate

prediction of the free surface elevation for rotational water waves using the recovery of pressure at the bed this paper considers the pressure streamfunction relationship for a train of regular water waves propagating on a steady current which may possess an arbitrary distribution of vorticity in two dimensions the application of such work is to both near shore and offshore environments and in particular for linear waves we provide a description of the role which the pressure function on the seabed plays in determining the free surface profile elevation our approach is shown to provide a good approximation for a range of current conditions this article is part of the theme issue nonlinear water waves

coherent doppler profiler measurements of near bed suspended sediment fluxes and the influence of bed forms 1 this paper reports on remote acoustic observations of vertical turbulence intensity and vertical suspended sediment flux profiles on a planar beach in 3 4 m water depth the measurements of suspended sediment concentration and velocity are colocated and simultaneous and extend through the wave bottom boundary layer to the bed with 0 7 cm vertical resolution normalized cospectra of the suspended sediment flux and the vertical velocity for different bed states irregular ripples cross ripples linear transition ripples and flat bed indicate a small but significant peak at incident wave frequencies but are otherwise rather flat with weak redness estimates of the vertical flux components indicate a general balance between upward fluxes due to waves and turbulence and downward settling two exceptions to this balance are found immediately above the bed and for nonmigrating irregular ripples the contribution from the high frequency turbulent component is small wave phase averages for low energy bed states exhibit near bed peaks in the suspended sediment flux following wave phase reversal wave phase averages for the high energy cases do not exhibit a diffusive signature observed vertical profiles of turbulence intensity for different bed states reveal that the near bed turbulence levels are relatively independent of bed state friction velocity predictions from presently available models including a bed stress model and a sediment eddy diffusion model are compared to measured values of near bed turbulence intensity reasonable agreement is found between measured and predicted bottom friction velocities when wave friction factors from tolman 1994 are used

can rogue waves be predicted using characteristic wave parameters rogue waves are ocean surface waves larger than the surrounding sea that can pose a danger to ships and offshore structures they are often deemed unpredictable without complex measurement of the wave field and computationally intensive calculation which is infeasible in most applications consequently there a need for fast predictors here we collate quality control and analyse the largest dataset of single point field measurements from surface following wave buoys to search for predictors of rogue wave occurrence we find that analysis of the sea state parameters in bulk yields no predictors as the subset of seas containing rogue waves sits within the set of seas without however spectral bandwidth parameters of rogue seas display different probability distributions to normal seas but these parameters are rarely provided in wave forecasts when location is accounted for trends can be identified in the occurrence of rogue waves as a function of the average seas state characteristics at that location these trends follow a power law relationship with the characteristic sea state parameters mean significant wave height and mean zero up crossing wave period we find that frequency of occurrence of rogue waves and their generating mechanism is not spatially uniform and each location is likely to have its own unique sensitivities which increase in the coastal seas we conclude that forecastable predictors of rogue wave occurrence will need to be location specific and reflective of their generation mechanism therefore given location and a sufficiently long historical record of sea state characteristics the likelihood of occurrence can be obtained for mariners and offshore operators

holocene fluctuations of lake levels in the french jura and sub alpine ranges and their implications for past general circulation patterns temperature and precipitation variations reflected in lake level fluctuations in the french jura and pre alps during the last nine millennia are qualitatively assessed by reference to 1 climatic oscillations reconstructed in the alps and 2 modern processes behind alpine tree limit variations the summer season and mainly the late summer appears to have a deterministic impact on the holocene environment correlations between palaeoclimatic proxy data recorded in the north atlantic area from high and middle latitudes and climatic dynamics illustrated by the little ice age and the recent warming suggest 1 the existence of climatic oscillations due to varying solar activity superimposed on larger plurimillennial climatic trends and 2 that these minor climatic phases correspond with two main types of general circulation pattern alternating over western europe during the last six millennia the climatic deteriorations correspond with cooling at high latitudes inducing a stronger thermal grad

isotope signature of the younger dryas and two minor oscillations at gerzensee switzerland palaeoclimatic and palaeolimnologic interpretation based on bulk and biogenic carbonates oxygen and carbon isotope ratios in the carbonate of benthic ostracodes pseudocandona marchica and molluscs pisidium ssp were measured across the transitions bordering the younger dryas chronozone in littoral lacustrine cores from gerzensee switzerland the specific biogenic carbonate records confirm the major shifts already visible in the continuous bulk carbonate oxygen isotope record 18occ if corrected for their vital offsets oxygen isotope ratios of pisidium and juvenile p marchica both formed in summer are almost identical to 18occ this bulk carbonate is mainly composed of encrustations of benthic macrophythes chara ssp also mainly produced during summer adult p marchica which calcify in winter show consistently higher 18o larger shifts across both transitions and short positive excursions compared with the summer forms especially during early preboreal despite such complexity the 18o of adult p marchica probably reflects more accurately the variations of the 18o of former lake water because during winter calcification temperatures are less variable and the water column isotopically uniform the difference between normalised 18o of calcite precipitated in winter to that formed in summer can be used to estimate the minimum difference between summer and winter water temperatures in general the results indicate warmer summers during the late allerod and early preboreal compared with the younger dryas altogether the isotopic composition of lake water 18ol and of the dissolved inorganic carbonate 13cdic reconstructed from adult pseudocandona marchica as well as the seasonal water temperature contrasts indicate that the major shifts in the 18o of local precipitation at gerzensee were augmented by changes of the lake s water balance with relatively higher evaporative loss occurring during the allerod compared with the younger dryas it is possible that during the early preboreal the lake might even have been hydrologically closed for a short period we speculate that such hydrologic changes reflect a combination of varying evapotranspiration and a rearrangement of groundwater recharge during those climatic shifts

quantitative climate reconstruction from pollen data in the grand duchy of luxembourg since 15 000 yr bp monthly temperature and precipitation estimates for the period 15000 2000 yr bp are statistically derived from pollen analytical data obtained from a site near echternach luxembourg a continuous warming trend of over 10 c is recorded for the period 10500 8000 yr bp along with a rise in precipitation of 500 mm a fairly constant temperature characterised the period between 8000 and 2000 yr bp though an episode of slight temperature decrease 2 c occurred at about 4500 yr bp the most humid intervals seem to have been at around 9000 7000 and 5000 3000 yr bp principally during the warm season more continental influence in contrast with results obtained from more southerly sites the older dryas climatic reversal is clearly recorded at echternach slightly before 12000 yr bp it is marked by a cooling in july temperature of about 4 c and a decrease in monthly precipitation of about 30 mm 40

velocity profiles above and within the wave bottom boundary layer over a sloping bottom theoretical solutions for the wave bottom boundary layer wbl are obtained using a viscoelastic turbulent closure model including sloping bed effects the overlying free stream velocity is given by nonlinear stokes wave theory for a sloping bottom the viscoelastic closure scheme extends conventional eddy viscosity models by incorporating the effects of eddy relaxation and diffusion on vertical momentum exchange in the wbl and gives improved predictions of observed velocity profiles field measurements of nearbed velocity profiles are then compared with theoretical solutions for a 2 bed slope field measurements were obtained in the lower 50 cm of the water column in 3 7 m depth with a coherent doppler profiler results are presented from a storm event for flat bed within the region 8 cm above the bed the observed velocity profiles change rapidly in amplitude and phase relative to potential flow theory indicating the presence of the wave boundary layer furthermore the observed vertical velocity profile in this region deviates significantly from the wbl theory for a horizontal flat bottom but compares well with the sloping bottom theory presented here