eemian cooling in the norwegian sea and north atlantic ocean preceding continental ice sheet growth changing conditions in the north atlantic region may drive global climate changes1 2 according to previous reconstructions of the last interglacial the eemian north atlantic sea surface tempera tures ssts were similar to present day values3 in the norwegian sea even warmer conditions appeared as a single pulse of short duration4 5 whereas the greenland ice record suggests that the warm interglacial air temperatures were interrupted by several cold periods6 here we use faunal and stable isotope analyses of foraminifera in two sediment cores from the north atlantic ocean and norwegian sea to reconstruct high resolution records of sst and sea surface salinity sss during the eemian interglacial our results which differ significantly from the greenland record6 show a sharp decrease in sst and sss of the norwegian sea associated with a more moderate cooling and freshening of the north atlantic at the middle of isotope substage 5e several millennia before the beginning of continental ice sheet growth changes in the norweg ian sea surface conditions appear to have represented an important climate change affecting global atmospheric and thermohaline circulations

fine grained versus coarse grained wave ripples generated experimentally under large scale oscillatory flow abstract wave ripples were generated in a wave tunnel under large scale oscillatory flow orbital diameter 1 4 5 m using two different grain sizes very fine sand and coarse sand the geometry of bed configurations that were produced varied strongly as a function of grain size small anorbital ripples wavelengths 10 cm heights 5 cm these results support the hypothesis advanced by leckie 1988 that large waves generate markedly different stratigraphic signatures in fine grained and coarse grained sediment

high resolution record of climate stability in france during the last interglacial period the last interglacial period 127 110 kyr ago has been considered to be an analogue to the present interglacial period the holocene which may help us to understand present climate evolution but whereas holocene climate has been essentially stable in europe variability in climate during the last interglacial period has remained unresolved because climate reconstructions from ice cores1 2 continental records3 4 and marine sediment cores5 6 give conflicting results for this period7 here we present a high resolution multi proxy lacustrine record of climate change during the last interglacial period based on oxygen isotopes in diatom silica diatom assemblages and pollen climate transfer functions from the ribains maar in france contrary to a previous study8 our data do not show a cold event interrupting the warm interglacial climate instead we find an early temperature maximum with a transition to a colder climate about halfway through the sequence the end of the interglacial period is clearly marked by an abrupt change in all proxy records our study confirms that in southwestern europe the last interglacial period was a time of climatic stability and is therefore still likely to represent a useful analogue for the present climate

clay mineral record in lake baikal sediments the holocene and late glacial transition abstract we analyse the clay mineralogical composition of two 1 m long cores from elevated plateaus in the southern and northern basins of lake baikal our aim is to test the limits of classically used clay proxies for climate reconstructions mineralogical signature is determined by x ray diffraction on oriented aggregates from holocene and late glacial sediments mineralogical results are presented on a palaeomagnetic derived time scale sampling at a centimeter resolution allows for a sub millenial order reconstruction the evolution of clay derived climate proxies smectite illite peak intensity ratio s i is compared between the two locations with respect to other climate reconstructions for the lake baikal area and siberia during the late glacial punctual increases of s i are related to changes of particle source rather than any climate change at both sites the most intense chemical weathering conditions occured during the subboreal lagging the eurasian middle atlantic climate optimum s i follows a gradual but irregular increase through the holocene probably related to slow warming favourable to the development of the siberian soils the different sensitivity to hydrolysing conditions in northern and southern sites can be explained by a combination of local lithological and topographical parameters

late quaternary clay mineral record in central lake baikal academician ridge siberia abstract we investigated the mineralogical composition of two cores recovered on the academician ridge central lake baikal siberia sedimentological features show that the cores are unaffected by turbidity currents however hemipelagic deposition is not continuous but intermittently disturbed by syn or post sediment reworking e g bioturbation slumps faulting such modes of deposition are consistent with the complex uplift history of the ridge bulk mineralogy suggests that terrigenous sediment supplies are constant through glacial interglacial stages and diluted by diatom rich intervals related to warmer interglacial stages the core stratigraphy is based on the correlation of the diatom zonation and opal abundance with the marine oxygen isotope reference curve specmap the 8 m cores partly recover the last four interglacial glacial cycles i e since oxygen isotope stage 8 we test the use of clay minerals as a proxy for paleoclimatic reconstruction the clays are more weathered during the diatom rich intervals in agreement with warmer climate conditions however the mean clay composition does not change significantly through glacial interglacial stages this observation implies that in the academician ridge sediments a simple smectite illite ratio s i does not alone provide a reliable indicator of climatic variation it reflects the complex clay assemblages especially the smectite group delivered to central lake baikal smectites include primarily illite smectite mixed layers made of a mixture of montmorillonite and beidellite according to their behavior after cation saturation the illite smectite mixed layers are primarily transformed smectites with some neoformed smectites intermittently observed in addition al smectites occur in minor proportions we conclude that the s i ratio has a climatic significance only if it evolves in parallel with the weathering stage of the clays and is confirmed by a change in the composition of the smectites

weathering in the lake baikal watershed during the kazantsevo eemian interglacial evidence from the lacustrine clay record the clay mineralogical record of a piston core recovered on an elevated plateau in the northern basin of lake baikal has been investigated for the kazantsevo interglacial period i e eemian s s equivalent in northern europe the age model as inferred from palaeomagnetic intensity suggests that this stage spans ca 128 to 117 kyr bp relative clay mineral abundances and clay mineral ratios are used to reconstruct the weathering conditions within the baikal watershed at a sub millennial resolution and suggest that the clay record is highly variable a bimodal clay data distribution is in agreement with different clay sources and or formation between the studied glacial and interglacial periods high amounts of smectites in the taz glacial samples 128 7 136 4 kyr bp may be explained by an additional source of neoformed smectites during the glacial stage in addition to the classically used smectite illite ratio the mineralogical results are integrated by the calculation of a hydrolysis index that takes into account the abundance of all clay species and their sensitivity to chemical weathering a principal components analysis pca of the baikal clay minerals allows the comparison of the clay parameters with regard to weathering conditions clay data are further compared i with diatom and pollen profiles ii with pollen based quantitative reconstructions for the same core material and iii with other climate reconstructions for the lake baikal region and siberia several features of our record are highlighted here during the early period of the kazantsevo interglacial 128 4 125 2 kyr bp weathering processes remain controlled by physical reworking for more than 2 kyr after the initial transition from cold to warm conditions inception of chemical weathering starts only after ca 125 kyr bp a period coincident with the warmest conditions according to both the pollen record and by the strongest chernozem development in siberian soils within the interglacial interval the hydrolysis index displays a two step increase punctuated by a minimum value ca 122 kyr bp the increasing but irregular trend persists after the transition from the kazantsevo interglacial to the zyryanka glacial similar to 117 kyr bp peak chemical weathering as inferred by clay changes lags the interglacial glacial transition by at least 2 kyr this suggests that pedogenesis remains active after the interval of surface stabilization lake baikal clay minerals trace the nature of the main weathering conditions within the watershed we note any increase in physical weathering is rapidly recorded in sedimentary clay assemblages but the mineral imprint to chemical weathering changes is more gradual lagging reconstructed climate conditions over the lake by ca 2 kyr 0 2007 elsevier b v all rights reserved

late warming and early cooling of the sea surface in the nordic seas during mis 5e eemian interglacial abstract geochemical identification of a tephra layer found in two cores from the ne atlantic ocean and the se norwegian sea respectively and dated to 127 ka bp has enabled us to obtain a precise correlation across the iceland scotland ridge at the marine isotope stage mis 6 5 transition the direct distance between the two cores is only about 200 km south of the iceland scotland ridge sea surface temperatures rose abruptly at 130 ka bp at the onset of mis 5e and at least 2 3000 years earlier than north of the ridge maximum sea surface temperatures south of the ridge occurred during this initial phase of mis 5e when temperatures in the nordic seas were still low north of the ridge the sea surface warmed rapidly at 127 ka bp correlations between the north atlantic records and the eemian of northwest europe tentatively indicate that the initial phase of mis 5e correlates with the early part of the eemian characterised by a warm continental type of climate the period after the warming of the nordic seas corresponds to the slightly cooler and more oceanic middle eemian interval in europe the sea surface temperatures fell gradually north of the ridge during the later part of mis 5e and they were low during mis 5d 5a south of the ridge the temperatures remained relatively high the data shows that there was no outflow of deep water from the norwegian sea during the later part of mis 6 outflow began at the mis 6 5 transition simultaneous with the sea surface warming south of the ridge

geometry prediction for wave generated bedforms published data on wave generated bedform geometry from field and laboratory measurements are re evaluated in terms of relative fluid orbit diameter at the bed and a wave period parameter d 8t2 laboratory bedform data shows a strong dependence on these parameters however for very small values of the period parameter typically field data there is evidence that bedform length depends only on sediment grain size the disappearance of sediment bedforms at high fluid velocities apparently occurs through a decrease in bedform height improved methods for the prediction of bedform geometry under waves are provided in graphical form and computer code

climate variations in europe over the past 140 kyr deduced from rock magnetism rapid shifts in climate during the last glacial are now well documented particularly from the oxygen isotope records of the two greenland ice cores grip1 2 and gisp23 in the grip record1 2 these climate events are also seen during the preceding eemian interglacial which may be an analogue for the future climate warmed by the greenhouse effect but these shifts are not found in the eemian section of the gisp2 core3 casting doubt on whether the rapid shifts in the grip oxygen isotope record really do represent a climate signal here we present magnetic susceptibility pollen and organic carbon records from maar lake deposits in the massif central france these data provide an independent record of past climate and we find that they correlate well with the ice core records during the last glacial during the eemian two rapid cooling events seen in our record also correlate with those seen in the grip ice core supporting the idea that rapid climate change did occur in the eemian interglacial and demonstrating that it extended to continental europe

a high resolution diatom record of the palaeoclimates of east siberia for the last 2 5 my from lake baikal abstract two parallel drilling cores bdp96 1 200 m and bdp96 2 100 m have been taken from top of the underwater akademichesky ridge in lake baikal and dated by paleomagnetic techniques these cores are part of the baikal drilling project kuzmin et al 1997a b diatom analysis of the first 100 m of these sediments reveals that they store a 2 5 myr high resolution continuous record of the palaeoclimates of east siberia sediments belonging to warmer climates have a high content of diatom algae frustules whereas those belonging to global glaciations are diatom barren the record of upper pleistocene 500 kyr bp strongly correlates with the oceanic specmap curve and contains distinct 100 41 23 and 19 kyr periods as revealed by fourier analysis diversity of diatom species was small in pliocene abrupt and frequent changes of the climate in pleistocene resulted in frequent dramatic changes in the diatom communities the extant dominating endemics cyclotella minuta and aulacoseira baicalensis became important in lake baikal 760 kyr and less than 120 kyr bp respectively

clay minerals in the sediments of lake baikal a useful climate proxy lake baikal in south central siberia has been the focus of an international effort the baikal drilling project bdp to obtain continuous long cores upwards of 100 m from this unique rift valley lake and to interpret the paleoclimatic history from various proxy data as part of this effort the clay minerals were examined by two research teams a consistent clay mineral assemblage containing illite inter stratified illite smectite chlorite and kaolinite as the major minerals characterizes much of the modern sediments the relative abundance of these minerals changes with depth in both short piston cores from various parts of the lake and in 100 m long cores taken from the distal toe of the selenga delta bdp 93 independent analyses of the abun dance changes and correlation with other data from the cores show that the clays are responding to two influences 1 climatic fluctua tions particularly in the upper 40 m of the sedimentary record which show the relative amount of illite smectite and sometimes kaolinite increasing during warmer climate episodes and 2 source area chang es which are most evident below 40 m in the bdp 93 cores and mark a shift from an eastern buguldeika river to a western selenga river source the clay mineral based climate fluctuations are correlative with the marine oxygen isotope record through stage 7 and provide a relatively simple and cost effective tool for gaining insight into the pa leoclimate of this interior continental site

geomagnetic paleointensity and directional secular variation at ocean drilling program odp site 984 bjorn drift since 500 ka comparisons with odp site 983 gardar drift normalized remanence relative geomagnetic paleointensity records for the last 500 kyr can be matched between two iceland basin ocean drilling program odp sites 984 and 983 and correlated with other high resolution records directional secular variation is however not easily correlated between sites due to inadequate recording at these mean sedimentation rates 12 15 cm kyr of the characteristic high frequency variability of the directional record both sites record the iceland basin event at 186 189 ka in which the characteristic magnetization component rotates through 180 and back coincident with a paleointensity low that lasted about 3 kyr other brunhes chron geomagnetic excursions appear to be manifest as intervals of higher amplitude secular variation during lows in paleointensity there is a tendency for paleointensity lows to correlate with peak interglacials in the oxygen isotope records however this does not translate into a correlation between paleointensity and percent carbonate or between paleointensity and magnetic concentration or grain size parameters

palaeoenvironmental history of lake baikal during the last 23000 years abstract a sediment core recovered from lake baikal was studied in an attempt to elucidate the palaeoenvironmental history of an intra continental lake under the control of last glacial to post glacial climatic fluctuations the lake conditions were characterised by subdued primary production and increased turbidity conditions during the last glacial maximum terrestrial vegetation and the development of soil layers were reduced in the drainage areas the deglaciation suggested by increased terrigenous sand influx and biological productivity took place at around 18 000 yr b p responding to the increased insolation in the northern hemisphere high latitude the onset of wet and warm conditions at ca 12 000 yr b p brought on a maximum of aquatic production and forestal expansion between 7 000 and 6 000 yr b p since 5 000 yr b p enhanced blooming of diatoms has occurred due to an increased nutrient supply and dissolved silica input into the lake

a unified model for bedform development and equilibrium under unidirectional oscillatory and combined flows the development of bedforms under unidirectional oscillatory and combined flows results from temporal changes in sediment transport flow and morphological response in such flows the bedform characteristics for example height wavelength and shape change over time from their initiation to equilibrium with the imposed conditions even if the flow conditions remain unchanged these variations in bedform morphology during development are reflected in the sedimentary structures preserved in the rock record hence understanding the time and morphological development in which bedforms evolve to an equilibrium stage is critical for informed reconstruction of the ancient sedimentary record this article presents results from a laboratory flume study on bedform development and equilibrium development time conducted under purely unidirectional purely oscillatory and combined flow conditions which aimed to test and extend an empirical model developed in past work solely for unidirectional ripples the present results yield a unified model for bedform development and equilibrium under unidirectional oscillatory and combined flows the experimental results show that the processes of bedform genesis and growth are common to all types of flows and can be characterized into four stages i incipient bedforms ii growing bedforms iii stabilizing bedforms and iv fully developed bedforms furthermore the development path of bedform growth exhibits the same general trend for different flow types for example unidirectional oscillatory and combined flows bedform size for example small versus large ripples bedform shape for example symmetrical or rounded bedform planform geometry for example two dimensional versus three dimensional flow velocities and sediment grain sizes the equilibrium time for a wide range of bed configurations was determined and found to be inversely proportional to the sediment transport flux occurring for that flow condition

wave formed sediment ripples transient analysis of ripple spectral development 1 a new method has been developed that models the changes a wave formed rippled sediment bed undergoes as it is actively evolving between two given equilibrium states due to a change in surface wave conditions the transient analysis of rippled beds has received very little attention within the literature dynamic changes within ripple parameters have implications for the estimation of flow dissipation and sediment transport by changing the bottom roughness height the method uses the spectral density function of the rippled bed and is based on a series of ripple growth and ripple transition experimental tests the ripple evolution model was developed from the well known logistic growth law fitting the general solution of the logistic nonlinear differential equation to the experimental data enabled the evolution rate of the bed to be determined for each experimental test it was concluded that there was no difference between the evolution rate determined from the ripple growth tests and the ripple transition tests this indicated that the two types of growth are special cases of the same evolution processes which is adequately modeled by the logistic growth equation a functional dependence was established between the ripple evolution rate and the shields parameter this allows the evolution rate to be estimated from flow and sediment properties the estimation of the rate at which rippled sediment beds evolve under a variable sea state has the potential to lead to significant improvements to the way ripple transition and hence bottom roughness is approximated in coastal wave models

oscillatory ripple geometry ripple geometry provides a bed roughness which governs the bed shear stress and the profile of eddy viscosity or diffusion coefficient required for the description of the sediment concentration profile or rate of sediment transport an analysis is made of ripple geometries using known tabulated laboratory and field data as well as water tunnel data obtained by the writer three different types of ripples can be identified sand light sediment and oscillating bed ripples they reflect different effects of the fluid shear stress and fluid acceleration on bed sediments it is also shown that when the bed shear stress or the amplitude of the water particle motion relative to the sediment size is small the resulting vortices are more regular with low turbulence intensity and the generated ripples are two dimensional they are called growing or vortex ripples which are strongly influenced by the amplitude of the water particle motion at the other extreme when the bed shear stress or the amplitude of the

a late eemian aridity pulse in central europe during the last glacial inception how do ice ages begin it s an obvious question to ask as we enjoy the relative luxury of an interglacial but a hard one to answer a look at past transitions may give some clues as to how this period will one day come to an end a climate reconstruction based on sediments found beneath a lake in the eifel mountains in germany provides evidence of an extreme climate event lasting 468 years right at the end of the last interglacial dust storms aridity bushfires and the loss of trees associated with a warm climate coincided with a southward shift of the warm waters of the north atlantic drift in terms of insolation the rate of delivery of the sun s radiation to earth conditions then were similar to those we have now investigating the processes that led to the end of the last interglacial period is relevant for understanding how our ongoing interglacial will end which has been a matter of much debate see for example refs 1 2 a recent ice core from greenland demonstrates climate cooling from 122 000 years ago3 driven by orbitally controlled insolation with glacial inception at 118 000 years ago4 5 6 7 8 here we present an annually resolved layer counted record of varve thickness quartz grain size and pollen assemblages from a maar lake in the eifel germany which documents a late eemian aridity pulse lasting 468 years with dust storms aridity bushfire and a decline of thermophilous trees at the time of glacial inception we interpret the decrease in both precipitation and temperature as an indication of a close link of this extreme climate event to a sudden southward shift of the position of the north atlantic drift the ocean current that brings warm surface waters to the northern european region the late eemian aridity pulse occurred at a 65 n july insolation of 416 w m 2 close to today s value of 428 w m 2 ref 9 and may therefore be relevant for the interpretation of present day climate variability

highest natural bed forms 1 fluid flow interacts with sedimentary beds forming waves of different kinds which are of considerable practical importance since they influence significantly the near bed flow both over and below the bed sediment transport and wave height attenuation we focus here on steep bed forms capable of producing flow separation in this case the large scale vorticity generated in the phenomenon of separation rules the process of friction which appears to be practically unaffected by sediment motion under the crests of the bed forms the mean shear force due to friction is balanced by the force that bed forms exert on the flow via pressure which can be calculated from the work of gimenez curto and corniero 2002 bed forms grow until they have a height such that friction at their troughs is negligible thus ceasing the motion of fluid and sediment this condition leads to a very simple expression for the limiting steepness which compares favorably with existing observations on bed form geometry under steady open channel flow as well as under oscillatory flow

numerical study of sediment transport above rippled beds under the action of combined waves and currents to study sediment suspension above ripples under the combined action of waves and currents a three dimensional numerical model has been developed based on the use of fluent software and an external sediment transport model the computer model has been tested against laboratory measurements involving oscillatory wave motion as well as cases of co linear waves with following and opposing currents with satisfactory results compared with the situation in which only waves are present called waves alone cases the effects from the steady current on both vortex shedding and sediment suspension above ripples have clearly been revealed by the model results in particular the vortices generated in combined waves and currents tend to stay low in the trough area of the ripple and are ejected earlier than those in the waves alone case at both the ripple crest and trough which leads to concentration peaks at different phases and with different magnitudes the model was also applied to a field case from a multi barred dissipative beach at egmond an zee in the netherlands to investigate the influences of a long shore current on cross shore sediment transport the model results show reasonable overall agreement with the field measurements as well as the important effects of the three dimensional flow structure on the sediment entrainment process close to the ripple surface which is very difficult to observe in such detail in field studies

the siberian loess record and its significance for reconstruction of pleistocene climate change in north central asia abstract the siberian loess represents the most significant terrestrial proxy record of quaternary climatic history in northern asia loess is extensive in southern siberia between the irtysh basin in the west and the angara basin in the east palaeoenvironmental multi proxy data magnetic susceptibility ms grain size caco 3 toc pollen etc from high resolution loess palaeosol sections provide evidence for a strongly fluctuating climatic sequence in north central asia during the late quaternary displaying the globally diagnostic sequences found in the deep sea oxygen isotope records ms provides the most continuous and detailed record encompassing the last two interglacial glacial cycles in loess sections in the minusinsk basin yenisei area and on the northern altai plains evidence for periodic atmospheric shifts is provided by gradual changes in the main vegetation zones with parkland steppe and mixed taiga during warm interglacial interstadial stages being replaced by boreal tundra forest and arid periglacial tundra steppe during stadials climatic pulses from warm intervals to cold are recorded by incipient forest steppe tundra gleysols the occurrence of the highest sediment accumulation rates in glacial stages ois 4 and ois 2 in the southern plains and depressions indicates that the most intensive aeolian dust deposition followed the glacial maxima with the most recent interval dated to ca 19 15 ka bp because of its zonal distribution and pronounced climatic continentality the siberian loess region represents an important data source for reconstructing past climates in the northern hemisphere it provides a key to the correlation of other loess regions within the west east eurasian continental zone

detailed measurements of velocities and suspended sand concentrations over full scale ripples in regular oscillatory flow the knowledge and modeling of wave induced sand transport over rippled beds still has significant shortcomings which is partly related to a lack of measurements of the detailed processes from controlled laboratory experiments we have carried out new measurements of the detailed time dependent velocity and suspended sand concentration field around vortex ripples for regular oscillatory flow conditions the fact that the ripples were mobile and the flow conditions were full scale makes these measurements unique we made velocity measurements for 14 different flows and concentration measurements for three of these flows the velocity and concentration field above ripples are dominated by the generation and ejection of vortices on the ripple flanks around the time of flow reversal vortex formation results in near ripple flow reversals ahead of free stream reversals and velocity maxima near the ripple crest that are much higher than the free stream maxima asymmetry in the free stream produces steady circulation cells with dominant offshore mean flow up the ripple lee slope balanced by weaker onshore streaming up the ripple stoss slope as well as higher up in the flow the time and bed averaged horizontal velocity profile comprises an offshore streaming near the bed and an onshore drift higher up in the flow the vortices are responsible for three main concentration peaks one just after on offshore flow reversal associated with the passage of a sand laden vortex followed by two smaller peaks due to advected suspension clouds generated by vortex action at the neighboring onshore ripples the sand flux field measured for one typical asymmetric flow condition is dominated by an offshore flux associated with the suspended sand cloud generated by vortex shedding from the ripple s lee slope around the time of on offshore flow reversal the net time averaged current related and wave related horizontal sand fluxes are generally offshore directed and mostly contained within 1 5 ripple heights above the ripple crest the wave related suspended transport component is larger but the contribution of the current related suspended sand transport cannot be neglected in addition to the measured offshore net transport of suspended sand there is an onshore directed transport very close to the ripple surface the total net transport is in the offshore direction for this specific asymmetric flow condition

the stability and the abrupt ending of the last interglaciation in southeastern siberia abstract paleoproductivity records from lake baikal indicate that the last interglaciation in southeastern siberia lasted ca 12 000 yr from 127 000 to 115 000 yr b p was punctuated by a climatic oscillation about 120 000 yr ago and ended abruptly with a rapid onset of regional glaciation during marine isotope substage mis 5d the siberian equivalent of the mis 5e 5d transition appears to lead the post eemian cold pulses in europe and in the central north atlantic by 5000 to 8000 yr the baikal record also registers a pronounced cold event during early mis 5c which correlates with the montaigu event in pollen records from france in siberia the climatic deterioration during the mis 5d was a full scale glacial pulse unlike the short moderate cooling during the montaigu event

millennial scale climatic oscillations during the last interglaciation in central china repeated southward excursions of north atlantic polar water during the last interglaciation 18 o stage 5 130 74 ka are recorded by planktonic foraminifera and ice rafted detritus in north atlantic sediment cores and greenland ice sheet cores display quasi synchronous fluctuations comparable high frequency variations in the east asian winter monsoon climate are discernible in three loess paleosol profiles in central china that span the last interglaciation peak values of the 40 m quartz fraction and bulk sediment samples from the s1 last interglacial accretionary paleosol complex reflect major dust flux events when winter monsoon winds strengthened frequent oscillations of the dust flux and nine significant dust events are recorded six events falling between ca 110 and 70 ka are correlated with cold peaks c19 24 identified in north atlantic cores two comparable dust peaks occur within paleosol s1ss3 substage 5e the older of these dating to ca 121 ka may correlate with a brief cold event recently recognized in high resolution marine and terrestrial climate proxy records

sediment transport in oscillatory boundary layers in cases of rippled beds and sheet flow an experimental study was focused on the process of sediment transport in unsteady flow conditions due to wave action wave induced oscillatory flow conditions near the seabed were simulated at full scale 1 1 in a new large oscillating water tunnel two sets of experiments series a and b were carried out during series a bed forms and wave cycle averaged suspended sediment concentrations were measured under sinusoidal waves series b focused on measurement of the wave cycle averaged sediment transport rates under regular and random asymmetric oscillatory flows upper shoreface conditions and was aimed at the verification of quasi steady formulas for the description of cross shore sediment transport bedform dimensions appeared to decrease considerably under the influence of wave asymmetry and wave randomness only the measured ripple dimensions under asymmetric waves series b showed good agreement with nielsen s 1979 relations for most of the series b experiments the bed was plane sheet flow the net sediment transport was directed onshore and the measured transport rates showed a strong correlation with the velocity moment an empirical quasi steady transport model is proposed which is based on the new tunnel data and other existing data sets the limitations of the quasi steady model approach became clear in the rippled bed regime and through the presence of a consistent influence of the wave period in plane bed conditions in rippled bed conditions the suspended concentration profiles followed a negative exponential distribution for most of the experiments the measured concentration decay length showed a linear relation with the ripple height in plane bed sheet flow conditions the measured suspended concentration profiles followed a negative power function the power or concentration decay parameter was constant 2 1 for a wide range of velocity conditions it is suggested that the mobile bed sheet flow layer has a strong damping effect on the mixing of suspended sediments turbulence damping

geometry migration and evolution of wave orbital ripples at leo 15 observations of the temporal evolution of the geometric properties and migration of wave formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements six weeks of bedform observations were taken at the sandy medium to coarse sized sand leo 15 site located on beach haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar during this period six tropical storms several of hurricane strength passed to the east of the study site ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed the predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3 4 of the wave orbital diameter although orbital diameters become larger than 130 cm during the maximum wave event it is unclear if a transition to non orbital scaling is occurring ripple migration is found to be directed primarily onshore at rates of up to 80 cm day suspended transport due to wave motions calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near bottom suspended transport bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave to current velocities

the role of the deep ocean in north atlantic climate change between 70 and 130 kyr ago the suggestion1 that changes in north atlantic deep water nadw production are linked through surface heat flux to the atmospheric temperature over greenland is supported by earlier indications2 3 that nadw production decreased during glacial times and by the subsequent finding4 6 that it declined during the younger dryas cool period at the end of the last glaciation changes in north atlantic surface temperatures have been found7 to mirror high frequency temperature changes recorded in greenland ice cores over the past 80 kyr but the connection to abyssal circulation has yet to be established except for one or two isolated oscillations8 9 here we present carbon and oxygen isotope analyses of benthic foraminifera in a high resolution north atlantic deep sea sediment core for the period 70 130 kyr ago these data allow us to reconstruct the history of nadw production which shows a close correlation with greenland climate variability for much of this time interval suggesting that the climate influence of nadw variability was widespread we see no evidence however for changes in nadw production during substage 5e the eemian interglacial period in contrast with recent ice core data10 which suggest severe climate instability in greenland during this time period our results may support suggestions based on data from a second ice core that this apparent instability is an artefact caused by ice flow11 alternatively the eemian climate instability may have had a different origin from the subsequent climate events

response of sand ripples to change in oscillatory flow ripples take time to evolve to a new equilibrium state in response to a change in wave generated oscillatory flow the paper presents results from flow tunnel experiments designed to examine oscillatory flow transient ripple processes under controlled full scale laboratory conditions the experiments include study of the growth of ripples from flat bed and the evolution of existing ripples to new equilibrium ripples in response to a step change in the flow in general ripples evolve through a combination of two main processes i from a flat bed or from a bed consisting of ripples that are smaller than the equilibrium ripples through a combination of slide and merge ii from a bed consisting of ripples that are larger than the equilibrium ripples through a combination of split and merge the experimental results show that equilibrium ripple geometry is independent of initial bed morphology while the time to reach equilibrium is largely independent of the initial bed and the equilibrium ripple size the time to reach equilibrium depends strongly on the mobility number and a new empirical equation relating mobility number and the number of flow cycles to equilibrium is proposed this equation is combined with a simple exponential function for ripple height growth or decay to produce a new empirical model for ripple height evolution which gives a reasonably good overall agreement with the measurements the model is based on experiments involving one sediment size only and further work is needed to develop the model for other sand sizes

geometric characteristics of coarse sand ripples generated by oscillatory flows a full scale experimental study abstract the shape of vortex ripples generated by wave boundary layers determines the local hydrodynamics and sediment motions and therefore is critical for understanding coastal sediment transport in the rippled bed regime in this study coarse sand vortex ripples were generated by periodic oscillatory flows mimicking full scale field conditions in an oscillatory water tunnel a laser based bottom profiler was applied for measuring the movable bed s profile the observed ripple development from a flat bed is in agreement with many previous studies small ripple marks appeared within a couple of flow periods and gradually became 3 dimensional 3 d transient ripples which eventually evolved to equilibrium 2 dimensional 2 d ripples after o 100 1000 flow periods the shape of equilibrium 2 d ripple was studied based on a representative ripple profile obtained by ensemble averaging a periodic ripple train the shape of ripples formed under sinusoidal flows become increasingly sinusoidal as the flow becomes stronger larger shields parameter or wave period becomes longer which is possibly because the influence of coherent vortex becomes more significant wave nonlinearities have different effects on the ripple shape generally speaking wave skewness makes ripples onshore leaning while wave asymmetry makes ripples crests sharper and troughs flatter our measurements also revealed some detailed geometric characteristics i e the roundness of ripple crests and the maximum local slope along ripple flanks

lake baikal record of continental climate response to orbital insolation during the past 5 million years the sedimentary record of biogenic silica from lake baikal in south central siberia suggests that this region of central asia was impacted by two major cooling episodes at 2 8 to 2 6 and 1 8 to 1 6 million years ago the spectral evolution of this continental interior site parallels the evolutionary frequency spectra for various marine oxygen isotope records in the baikal record the 41 000 year obliquity cycle is particularly strong from 1 8 to 0 8 million years ago variance in the 100 000 year eccentricity band increases during the past 0 8 million years the expected precession frequency of 23 000 years is highest during the past 400 000 years the modulation of the predicted 23 000 and 41 000 year insolation forcing by the 100 000 and 400 000 year eccentricity bands indicates that the transfer of variance from the precession and obliquity frequencies to the eccentricity part of the spectrum occurred in the eurasian continental interior as well as in tropical and high latitude ocean sites

the meaning of stromatolites stromatolites document microbial interactions with sediments and flowing water throughout recorded earth history and have the potential to illuminate the long term history of life and environments modern stromatolites however provide analogs to only a small subset of the structures preserved in archean and proterozoic carbonates thus interpretations of secular trends in the shapes and textures of ancient columnar stromatolites require nonuniformitarian scale dependent models of microbial responses to nutrient availability seawater chemistry influx of sediment grains shear and burial models that integrate stromatolite scales macroscopic organization and shapes could also help test the biogenicity of the oldest stromatolites and other structures whose petrographic fabrics do not preserve direct evidence of microbial activity an improved understanding of stromatolite morphogenesis in the presence of oxygenic and anoxygenic microbial mats may illuminate the diversity of microbial metabolisms tha

unifying large and small wave generated ripples 1 frequently in field and laboratory studies of wave generated bed forms on beds of sandy sediments both large and small wave generated ripples lwrs and swrs are observed and in some situations may be superimposed upon one another the present paper examines relationships between measured wave conditions and the geometry of lwrs and swrs and shows that existing empirical equations used to predict the dimensions of swrs perform badly for lwrs to address this problem relationships found between ripple geometry and the wave reynolds number are used as the basis for new expressions for prediction of ripple height wavelength and steepness that improve the accuracy of existing approaches for both lwrs and swrs the present results indicate strongly that swrs and lwrs are related and their superposition in certain conditions reflects the time lag between the response of the bed to changing hydrodynamic conditions they must be considered therefore when estimating the total bed roughness in a range of practical engineering and numerical modeling applications

bed forms created by simulated waves and currents in a large flume 1 the morphology and evolution of bed forms created by combinations of waves and currents were investigated using an oscillating plate in a 4 m wide flume current speed ranged from 0 to 30 cm s maximum oscillatory velocity ranged from 20 to 48 cm s oscillation period was 8 s except for one run with 12 s period and the median grain size was 0 27 mm the angle between oscillations and current was 90 60 or 45 at the end of each run the sand bed was photographed and ripple dimensions were measured ripple wavelength was also determined from sonar images collected throughout the runs increasing the ratio of current to wave i e oscillatory velocity decreased ripple height and wavelength in part because of the increased fluid excursion during the wave period increasing the ratio of current to waves or decreasing the angle between current and waves increased the three dimensionality of bed forms during the runs ripple wavelength increased by a factor of about 2 the average number of wave periods for evolution of ripple wavelength to 90 of its final value was 184 for two dimensional ripples starting from a flat bed bed form orientations at the end of each run were compared to four potential controlling factors the directions of waves current maximum instantaneous bed shear stress and maximum gross bed form normal transport mgbnt the directions of waves and of mgbnt were equally good predictors of bed form orientations and were significantly better than the other two factors

a model for the simulation of coupled flow bed form evolution in turbulent flows 1 we develop a three dimensional numerical model to simulate bed form dynamics in a turbulent boundary layer in the numerical model hydrodynamics is solved in a moving generalized boundary fitted curvilinear coordinate system such that the domain boundary exactly follows complex time dependent bed form geometry the resolved turbulent features are computed via large eddy simulation while the subgrid scale turbulent motions are modeled with a dynamic mixed model a second order accurate arbitrary lagrangian eulerian method is used to guarantee conservation of sediment mass while the grid moves arbitrarily due to the motion of the bed transport of suspended load is modeled using the eulerian approach with a pickup function as the bottom boundary condition for sediment entrainment at the bed transport of bed load and suspended load are combined in a mass balance equation for the bed which evolves due to the spatiotemporally varying bed stress induced by the turbulent flow field above the bed and gravity gravity induced avalanche flow motion of the bed in turn affects the flow field in a coupled hydrodynamic moving bed simulation in which bed features evolve due to resolved details of the turbulent flow we compare different bed elevation models and demonstrate the capability of the present model through simulation of sand ripple formation and evolution induced by turbulence in an oscillatory flow a resolution study demonstrates the need for fine grid resolution to resolve a bulk of the near wall turbulence which is essential for bed form initiation

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

quantitative reconstruction of the last interglacial vegetation and climate based on the pollen record from lake baikal russia changes in mean temperature of the coldest t c and warmest month t w annual precipitation p ann and moisture index were reconstructed from a continuous pollen record from lake baikal russia the pollen sequence con01 603 2 53 57 n 108 54 e was recovered from a 386 m water depth in the continent ridge and dated to ca 130 114 8 ky bp this time interval covers the complete last interglacial li corresponding to mis 5e results of pollen analysis and pollen based quantitative biome reconstruction show pronounced changes in the regional vegetation throughout the record shrubby tundra covered the area at the beginning of mis 5e ca 130 128 ky consistent with the end of the middle pleistocene glaciation the late glacial climate was characterised by low winter and summer temperatures t c 38 to 35 c and t w 11 13 c and low annual precipitation p ann 300 mm however the wide spread of tundra vegetation suggests rather moist environments associated with low temperatures and evaporation reconstructed 1 tundra was replaced by boreal conifer forest taiga by ca 128 ky bp suggesting a transition to the interglacial taiga dominant phase lasted until ca 117 4 ky bp e g about 10 ky the most favourable climate conditions occurred during the first half of the li p ann reached 500 mm soon after 128 ky bp however temperature changed more gradually maximum values of t c 20 c and t w 16 17 c are reconstructed from about 126 ky bp conditions became gradually colder after ca 121 ky bp t c dropped to 27 c and t w to 15 c by 119 5 ky bp the reconstructed increase in continentality was accompanied by a decrease in p ann to 400 420 mm however the climate was still humid enough 0 9 to support growth of boreal evergreen conifers a sharp turn towards a dry climate is reconstructed after ca 118 ky bp causing retreat of forest and spread of cool grass shrub communities cool steppe dominated the vegetation in the area between ca 117 5 ky and 114 8 ky bp suggesting the end of the interglacial and transition to the last glacial mis 5d shift to the new glaciation was characterised by cool and very dry conditions with t c 28 to 30 c t w 14 15 c p ann 250 mm and 0 5

ripple morphology under oscillatory flow 1 prediction 1 recent laboratory and field experiments have shown the inability of existing oscillatory flow ripple predictors to accurately predict both ripple size and ripple planform geometry however at this time only partial adaptations of these predictors have been proposed to account for the observed discrepancies with new experimental data the present work starts by presenting a comprehensive data set compiled from 27 literature studies and our own experimental data which are presented in a companion paper dimensional analysis is used both to clarify the importance of the different variables involved in the development of ripples and to generate the basic dimensionless framework for the study of ripple morphology under oscillatory flow several ripple size and planform geometry predictors are contrasted against the compiled data set afterward a new ripple wavelength and height predictor and a new planform geometry predictor are proposed which overcome several of the limitations found in the earlier formulations the new predictors emphasize the existence of local and global sediment transport mechanisms over a ripple bed each of them acting at different scales in particular sediment size is found to have an important effect on the size and geometry of bed forms that can be generated by oscillatory flows

sand ripples generated by regular oscillatory flow abstract the prediction of ripple geometry is a necessary precursor to the prediction of sand transport under waves for ripple regime conditions the paper begins with a comparison of four existing methods for predicting the geometry of sand ripples generated by oscillatory flow the comparison points to substantial differences between ripple dimensions predicted by the methods especially for field scale conditions ripple geometry experiments carried out in a large oscillatory flow tunnel are then described the experiments involved a range of sand sizes and sinusoidal and asymmetric flows with periods and velocities typical of field conditions comparison of measured and predicted ripple geometries leads to the recommendation that the method of mogridge davies and willis be used to predict ripple geometry for field scale oscillatory flows the nielsen method yields good predictions of ripple length but the rapid fall off in ripple steepness predicted by the nielsen method at high mobility number is not supported by the measurements the lengths and heights of symmetric ripples produced by sinusoidal flows are found to be similar to the lengths and heights of asymmetric ripples produced by equivalent asymmetric flows three dimensional ripples occur with fine sand in long period flows typical of field conditions the dimensions of these ripples cannot be predicted using methods developed for two dimensional ripples previously suggested criteria for predicting the occurrence of three dimensional ripples fail when tested against a wide range of flow and sand conditions the occurrence of three dimensional ripples and the effects of ripple and flow history on ripple geometry require further research

clays and secondary minerals as permafrost indicators examples from the circum baikal region abstract a proper understanding of quaternary environments requires assessment of the possible existence of perennially frozen ground furthermore the structural and mineralogical changes induced by permafrost are commonly irreversible and still affect the present soils so that it is important to be able to recognize them however the regions that suffered severe but dry periglacial conditions during cold quaternary periods as in southeastern siberia show few periglacial features five exposures investigated in the circum baikal area show permafrost features although the same phenomena occur throughout southeastern siberia these exposures demonstrate that even though evidence of permafrost may be lacking or equivocal thorough analysis can reveal indicators of permafrost such as neoformed clays secondary precipitates of iron calcite and sulphates with peculiar crystalline features as are found in other past or presently perpetually frozen ground in combination with sedimentary structures and the geomorphic environment these indicators affirm the definite existence of permafrost

spatial variability of the wave bottom boundary layer over movable rippled beds observations of the spatially dependent velocity field over movable bed forms subjected to slightly skewed and asymmetric regular wave forcing were collected the dynamics between the ripple elements is dominated by coherent vortices characterized by the swirling strength and evidenced in the temporal and spectral characterization within the boundary layer spectral energy in the second harmonic 3f0 is amplified at the ripple slopes and is consistent with the location of the expected strongest pressure gradients first moment and second moment velocity statistics were used to address the spatial variability of the intra ripple hydrodynamics estimates of displacement and momentum thicknesses and mom are smaller than suggested by the higher harmonics but consistently highlight areas of adverse and favorable pressure gradients shear stress and roughness estimates were inferred by fitting a logarithmic model to first moment and second moment statistics of the velocity field the maximum shields parameter was observed to peak at the stoss slope of asymmetric ripples during the strongest and shorter half wave period onshore first moment roughness estimates are similar in magnitude to bed load parameterizations provided by li et al 1997 and about a factor of 3 larger than second moment estimates assessment of the vertical transfer of horizontal momentum derived using a reynolds decomposition suggests that stresses inferred from the logarithmic law using first moment velocity statistics appropriately reproduce the mean momentum transfer for the longer and weaker offshore half wave period

suspended sediment transport and the role of infragravity waves in a barred surf zone abstract collocated optical backscatterance suspended solids sensors and electromagnetic flowmeters were deployed at stanhope lane beach p e i canada to assess the contributions of wind waves infragravity waves and mean currents to the cross shore flux of suspended sediment across a barred surf zone the local net transport of suspended sediment magnitude and direction was highly variable both spatially and temporally with respect to the relative magnitudes of the suspended sediment transport 1 that induced by quasi steady mean currents and oscillatory currents at both incident and infragravity frequencies were almost equal in magnitude when the total surf zone was averaged over the storm event 2 waves in the near infragravity band were quantitatively more important during the waning of the storm especially where when surf persisted well after the initial decay of the offshore wave field 3 incident waves were important where when the waves were not saturated i e during the early stages of the storm at all locations and persistently throughout the storm on the seaward slope of the bar where wave saturation was limited to a very short period with respect to the direction of net sediment transport 1 that attributable to mean currents was directed offshore in the inner surf zone probably reflecting the presence of an undertow while further seaward an onshore transport occurred probably resulting from mass transport induced by surf bores or drift velocities induced by the infragravity wave boundary layer 2 that induced by oscillatory currents at incident wave frequencies was generally onshore at any one location throughout the storm event in contrast oscillatory transport at low frequencies changed direction and also varied in magnitude at any given location in part these changes reflect the existence of significant oscillatory sediment transport at discrete infragravity frequencies the standing infragravity waves consistently transported suspended sediment towards antinodes in the water surface elevation